* src/gpgme.h.in (GPGME_SIG_MODE_FILE): New signature mode flag.
* src/engine-gpg.c (gpg_sign): Separate signature mode from additional
flags. Check for incompatible flags. Explicitly set output to stdout if
no output file is used. Pass filename instead of fd to gpg when new flag
is set.
* src/engine-gpgsm.c (gpgsm_sign): Return error if new flag is set.
* src/engine-uiserver.c (uiserver_sign): Ditto.
* src/sign.c (sign_start): Consider new flag on check for invalid flags.
* tests/run-sign.c (show_usage): New options --detach and
--direct-file-io.
(main): Parse new options. Create a detached signature if --detach is
given. Make gpg read the input file itself if --direct-file-io is given.