MDEV-10138 Support for decimals up to 38 digits

Decimals with float, double and decimal now works the following way:

- DECIMAL_NOT_SPECIFIED is used when declaring DECIMALS without a firm number
  of decimals.  It's only used in asserts and my_decimal_int_part.
- FLOATING_POINT_DECIMALS (31) is used to mark that a FLOAT or DOUBLE
  was defined without decimals. This is regarded as a floating point value.
- Max decimals allowed for FLOAT and DOUBLE is FLOATING_POINT_DECIMALS-1
- Clients assumes that float and double with decimals >= NOT_FIXED_DEC are
  floating point values (no decimals)
- In the .frm decimals=FLOATING_POINT_DECIMALS are used to define
  floating point for float and double (31, like before)

To ensure compatibility with old clients we do:

- When storing float and double, we change NOT_FIXED_DEC to
  FLOATING_POINT_DECIMALS.
- When creating fields from .frm we change for float and double
  FLOATING_POINT_DEC to NOT_FIXED_DEC
- When sending definition for a float/decimal field without decimals
  to the client as part of a result set we convert NOT_FIXED_DEC to
  FLOATING_POINT_DECIMALS.
- variance() and std() has changed to limit the decimals to
  FLOATING_POINT_DECIMALS -1 to not get the double converted floating point.
  (This was to preserve compatiblity)
- FLOAT and DOUBLE still have 30 as max number of decimals.

Bugs fixed:

variance() printed more decimals than we support for double values.

New behaviour:
- Strings now have 38 decimals instead of 30 when converted to decimal
- CREATE ... SELECT with a decimal with > 30 decimals will create a column
  with a smaller range than before as we are trying to preserve the number of
  decimals.


Other changes
- We are now using the obsolete bit FIELDFLAG_LEFT_FULLSCREEN to specify
  decimals > 31
- NOT_FIXED_DEC is now declared in one place
- For clients, NOT_FIXED_DEC is always 31 (to ensure compatibility).
  On the server NOT_FIXED_DEC is DECIMAL_NOT_SPECIFIED (39)
- AUTO_SEC_PART_DIGITS is taken from DECIMAL_NOT_SPECIFIED
- DOUBLE conversion functions are now using DECIMAL_NOT_SPECIFIED instead of
  NOT_FIXED_DEC

mysql-test/suite/sys_vars/r/div_precision_increment_basic.result

@@ -65,18 +65,18 @@ SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
 30
 '#------------------FN_DYNVARS_027_05-----------------------#'
-SET @@global.div_precision_increment = 31;
+SET @@global.div_precision_increment = 39;
 Warnings:
-Warning	1292	Truncated incorrect div_precision_increment value: '31'
+Warning	1292	Truncated incorrect div_precision_increment value: '39'
 SELECT @@global.div_precision_increment;
 @@global.div_precision_increment
-30
-SET @@global.div_precision_increment = 40;
+38
+SET @@global.div_precision_increment = 50;
 Warnings:
-Warning	1292	Truncated incorrect div_precision_increment value: '40'
+Warning	1292	Truncated incorrect div_precision_increment value: '50'
 SELECT @@global.div_precision_increment;
 @@global.div_precision_increment
-30
+38
 SET @@global.div_precision_increment = -1024;
 Warnings:
 Warning	1292	Truncated incorrect div_precision_increment value: '-1024'
@@ -88,19 +88,17 @@ Warnings:
 Warning	1292	Truncated incorrect div_precision_increment value: '65536'
 SELECT @@global.div_precision_increment;
 @@global.div_precision_increment
-30
-SET @@session.div_precision_increment = 40;
+38
+SET @@session.div_precision_increment = 50;
 Warnings:
-Warning	1292	Truncated incorrect div_precision_increment value: '40'
+Warning	1292	Truncated incorrect div_precision_increment value: '50'
 SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
-30
-SET @@session.div_precision_increment = 31;
-Warnings:
-Warning	1292	Truncated incorrect div_precision_increment value: '31'
+38
+SET @@session.div_precision_increment = 37;
 SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
-30
+37
 SET @@session.div_precision_increment = -2;
 Warnings:
 Warning	1292	Truncated incorrect div_precision_increment value: '-2'
@@ -112,27 +110,27 @@ Warnings:
 Warning	1292	Truncated incorrect div_precision_increment value: '65550'
 SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
-30
+38
 SET @@global.div_precision_increment = 65530.30;
 ERROR 42000: Incorrect argument type to variable 'div_precision_increment'
 SELECT @@global.div_precision_increment;
 @@global.div_precision_increment
-30
+38
 SET @@global.div_precision_increment = OFF;
 ERROR 42000: Incorrect argument type to variable 'div_precision_increment'
 SELECT @@global.div_precision_increment;
 @@global.div_precision_increment
-30
+38
 SET @@session.div_precision_increment = ON;
 ERROR 42000: Incorrect argument type to variable 'div_precision_increment'
 SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
-30
+38
 SET @@session.div_precision_increment = 65530.30;
 ERROR 42000: Incorrect argument type to variable 'div_precision_increment'
 SELECT @@session.div_precision_increment;
 @@session.div_precision_increment
-30
+38
 '#------------------FN_DYNVARS_027_06-----------------------#'
 SELECT @@global.div_precision_increment = VARIABLE_VALUE 
 FROM INFORMATION_SCHEMA.GLOBAL_VARIABLES