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

include/m_string.h

@@ -23,6 +23,7 @@
 #define _m_string_h
 
 #include "my_global.h"                          /* HAVE_* */
+#include "my_decimal_limits.h"
 
 #ifndef __USE_GNU
 #define __USE_GNU				/* We want to use stpcpy */
@@ -138,14 +139,13 @@ size_t my_fcvt(double x, int precision, char *to, my_bool *error);
 size_t my_gcvt(double x, my_gcvt_arg_type type, int width, char *to,
                my_bool *error);
 
-#define NOT_FIXED_DEC 31
-
 /*
   The longest string my_fcvt can return is 311 + "precision" bytes.
-  Here we assume that we never cal my_fcvt() with precision >= NOT_FIXED_DEC
+  Here we assume that we never cal my_fcvt() with
+  precision >= DECIMAL_NOT_SPECIFIED
   (+ 1 byte for the terminating '\0').
 */
-#define FLOATING_POINT_BUFFER (311 + NOT_FIXED_DEC)
+#define FLOATING_POINT_BUFFER (311 + DECIMAL_NOT_SPECIFIED)
 
 /*
   We want to use the 'e' format in some cases even if we have enough space