Failed exec resets job control state

[Grisha Levit]

An `exec` that can't execute the supplied command seems to cause the
status of job control to become reset to the default state:

   $ bash -O execfail -mc 'echo $-; exec xxx; echo $-'
   hmBc
   bash: line 0: exec: xxx: not found
   hBc

   $ set +m; echo $-
   hiBHs
   $ exec xxx
   bash: exec: xxx: not found
   $ echo $-
   himBHs