mpi: Improve mpi_invm to detect bad input.

* mpi/mpi-inv.c (mpi_invm): Return 0 for bad input.
--

Without this patch the function may enter an endless loop.  This is a
backport from libgcrypt.

GnuPG-bug-id: 1713
Allow use of --debug-level=LEVEL without '='.

* g10/gpg.c (opts): Fix "debug-level".
doc: Final update from master (gnupg 2.1)

* doc/Makefile.am (sources_from_trunk): Remove.
(update-source): Make it a dummy.
* doc/gpg.texi: Update.
* doc/yat2m.c: Update.
--

Maintaining 3 versions in of the gpg manual in one file is getting
more complicated with 2.1.  Thus we stop this now and keep the manual
for 1.4 separate.
doc: Cleanup gpg.texi.

--

We don't need the gpgone and gpgtwoone macros anymore.