sm: New option --ldapserver as an alias for --keyserver.

* sm/gpgsm.c (opts): Add option --ldapserver and make --keyserver an
alias.

We should use "keyserver" for OpenPGP and thus it is better to allow
for "ldapserver" here - it is the same convention as now used in
dirmngr.

• Signed-off-by: Werner Koch <wk@gnupg.org>