WL#1324 table name to file name encoding

- Encoding itself, implemented as a charset
  "filename". Originally planned to use '.'
  as an escape character, but now changed to '@'
  for two reasons: "ls" does not return
  file names starting with '.' considering them
  as a kind of hidden files; some platforms
  do not allow several dots in a file name.
- replacing many calls of my_snprintf() and
  strnxmov() to the new build_table_filename().
- Adding MY_APPEND_EXT mysys flag, to append
  an extention rather that replace it.
- Replacing all numeric constants in fn_format
  flag arguments to their mysys definitions, e.g.
  MY_UNPACK_FILENAME,
- Predictability in several function/methods:
  when a table name can appear with or withot .frm
  extension. Some functions/methods were changed
  so accept names strictly with .frm, other - strictly
  without .frm extensions. Several DBUG_ASSERTs were
  added to check whether an extension is passed.
Many files:
  table name to file name encoding
mysql_priv.h:
  Prototypes for new table name encoding tools.
ctype-utf8.c:
  Implementing "filename" charset for
  table name to file name encoding.
row0mysql.c:
  Fixing table name prefix.
mf_format.c:
  Adding MY_APPEND_EXT processing.
Many files:
  Fixing tests.
my_sys.h:
  Adding new flag to append rather than replace an extension.
m_ctype.h:
  Adding "filename" charset definition.

mysql-test/r/alter_table.result

@@ -373,10 +373,10 @@ t1	0	PRIMARY	2	User	A	0	NULL	NULL		BTREE
 t1	1	Host	1	Host	A	NULL	NULL	NULL		BTREE	disabled
 DROP TABLE t1;
 create table t1 (a int);
-alter table t1 rename to `t1\\`;
-ERROR 42000: Incorrect table name 't1\\'
-rename table t1 to `t1\\`;
-ERROR 42000: Incorrect table name 't1\\'
+alter table t1 rename to ``;
+ERROR 42000: Incorrect table name ''
+rename table t1 to ``;
+ERROR 42000: Incorrect table name ''
 drop table t1;
 drop table if exists t1, t2;
 Warnings: