Add new SVN only file README.maint

doc/

• gpg.texi (GPG Configuration): Document envvar LANGUAGE.

(GPG Configuration Options): Document show-primary-uid-only.
g10/

• gpg.c (main): Add verify option show-primary-uid-only.
• options.h (VERIFY_SHOW_PRIMARY_UID_ONLY): New.
• mainproc.c (check_sig_and_print): Implement it.

• encr-data.c (decrypt_data): Correctly test for unknown algorithm.
• import.c (check_prefs): Ditto.
• keyedit.c (show_prefs): Ditto.
• mainproc.c (proc_symkey_enc): Ditto.