(Ada) Add gdb-mi support for stopping at start of exception handler.

(Ada) Add gdb-mi support for stopping at start of exception handler.

Following my previous commit which add support for stopping at start of
exception handler, this commit adds required gdb-mi support for this
feature.

gdb/ChangeLog:

    * mi/mi-cmd-catch.c (mi_cmd_catch_handlers): New function.
    * mi/mi-cmds.c (mi_cmds): Add catch-handlers command.
    * mi/mi-cmds.h (mi_cmd_catch_handlers): Add external declaration.
    * NEWS: Document "-catch-handlers" command.

gdb/doc/ChangeLog:

    * gdb.texinfo (Ada Exception gdb/mi Catchpoints): Add
    documentation for new "-catch-handlers" command.

gdb/testsuite/ChangeLog:

    * gdb.ada/mi_catch_ex_hand.exp: New testcase.
    * gdb.ada/mi_catch_ex_hand/foo.adb: New file.

Tested on x86_64-linux.