Several functions (mostly in mysqld.cc) directly call
exit() function in case of errors, which is not a desired
behaviour expecially in the embedded-server library.

Fixed by making these functions return error sign instead
of exiting.

per-file comments:
  include/my_getopt.h
Bug#39289 libmysqld.a calls exit() upon error 
  added 'error' retvalue for my_getopt_register_get_addr

  libmysqld/lib_sql.cc
Bug#39289 libmysqld.a calls exit() upon error 
  unireg_clear() function implemented

  mysys/default.c
Bug#39289 libmysqld.a calls exit() upon error 
  error returned instead of exit() call

  mysys/mf_tempdir.c
Bug#39289 libmysqld.a calls exit() upon error 
  free_tmpdir() - fixed so it's not produce crash on uninitialized
    tmpdir structure

  mysys/my_getopt.c
Bug#39289 libmysqld.a calls exit() upon error 
  error returned instead of exit() call

  sql/mysql_priv.h
Bug#39289 libmysqld.a calls exit() upon error 
  unireg_abort definition fixed for the embedded server

  sql/mysqld.cc
Bug#39289 libmysqld.a calls exit() upon error 
  various functions fixed
  error returned instead of exit() call