Changed the creation of the .MRG file so that only the table  name
is written when the MyISAM table is in the same database as the
MERGE table, a relative path is used in other cases in mysqld,
and possibly an absolute path is used in an embedded server.
No test case is added as the external behaviour is unchanged.
Only the file names within the .MRG file are changed.


include/my_sys.h:
  BUG#5964 - 4.1 MERGE tables regression from 4.0
  Added declaration for a new function.
myisammrg/myrg_open.c:
  BUG#5964 - 4.1 MERGE tables regression from 4.0
  Changed check for absolute path to check for any path.
mysys/my_getwd.c:
  BUG#5964 - 4.1 MERGE tables regression from 4.0
  Added a new functions which checks for absolute _or_
  relative paths.
sql/ha_myisammrg.cc:
  BUG#5964 - 4.1 MERGE tables regression from 4.0
  Changed the creation of the .MRG file so that only the table  name
  is written when the MyISAM table is in the same database as the
  MERGE table, a relative path is used in other cases in mysqld,
  and possibly an absolute path is used in an embedded server.