Fix possible lockup in an atexit handler

* src/estream.c (_gpgrt_fflush): Add arg in_atexit and skip the flush in
  this mode if the lock can not be taken.  Change all callers.

The problem should up with gpgsm encryption on the standard mode and a
no public key error. In this case the process got stuck in the atexit
handler after a FAILURE gpgsm-exit status line to stderr. It is
better not to have a hanging process than having flushed everything.