decryption failed error if a MDC does not verify. Warn if a MDC is not
present (can disable via --no-mdc-warning).
* exec.c (exec_write), g10.c (main), keyserver.c (keyserver_spawn): Use
new DISABLE_KEYSERVER_PATH rather than FIXED_EXEC_PATH.
keyserver.c (keyserver_spawn): If the user does not use "exec-path",
completely replace $PATH with GNUPG_LIBEXECDIR before calling the
keyserver helper. If the user does use "exec-path", append
GNUPG_LIBEXECDIR after the specified path.
FIXED_PHOTO_VIEWER and DISABLE_PHOTO_VIEWER.
* mainproc.c (check_sig_and_print): Use --show-photos to show photos when
verifying a sig made by a key with a photo.
* keyserver.c (parse_keyserver_uri): Properly parse a URI with no :port
section and an empty file path, but with a terminating '/'.
(keyserver_work): Honor DISABLE_KEYSERVER_HELPERS.
* hkp.c (hkp_ask_import): Display keyserver URI as a URI, but only if
verbose.
* exec.c, g10.c: USE_EXEC_PATH -> FIXED_EXEC_PATH
