gpg: Warn about (but don't fail) on scdaemon options in gpg.conf.

* g10/gpg.c: Add config options that should belong in scdaemon.conf
* g10/main.h, g10/misc.c (obsolete_scdaemon_option): New.

--

In gpg2, the following options are only relevant for scdaemon:

 reader-port
 ctapi-driver
 pcsc-driver
 disable-ccid

but in gpg1, they are options for gpg itself.

Some users of gpg1 might have these options in their
~/.gnupg/gpg.conf, which causes gpg2 to fail hard if it reads that
config file.

gpg2 should not fail hard, though giving a warning (and suggesting a
move to scdaemon.conf) seems OK.

This patch does *not* reintroduce any documentation for these options
in gpg.texi, even to indicate that they are "dummy" options, since
scdaemon.texi contains the appropriate documentation.

Debian-bug-id: 762844

Program names factored out from obsolete_scdaemon_option to make
reuse without new translations easier. -wk

This is a backport of commit 371c2b14b0347209efd23b4e54e1981a12d7aeab
with parts of 20c6da50d4f6264d26d113d7de606971f719a0ca but without those
which would change existing translated strings. -wk
gpg: Add shortcut for setting key capabilities.

* g10/keygen.c (ask_key_flags): Add shortcut '='.
* doc/help.txt (gpg.keygen.flags): New.
doc: Update the file OpenPGP

--

It should actually be completey reworked but for now I added just a
few notes.
gpg: Default to SHA-256 for all signature types on RSA keys.

* g10/main.h (DEFAULT_DIGEST_ALGO): Use SHA256 in --gnupg and SHA1 in
strict RFC or PGP modes.
* g10/sign.c (make_keysig_packet): Use DEFAULT_DIGEST_ALGO also for
RSA key signatures.
--

(Backported from commit d33246700578cddd1cb8ed8164cfbba50aba4ef3)