* common/homedir.c (enum wantdir_values): New enums.
(unix_rootdir): Change arg to use the enums.  Adjust all callers.  Add
support for the socketdir keyword.
(_gnupg_socketdir_internal): Take care of the socketdir keyword in
gpgconf.ctl.
* doc/tools.texi (Files used by gpgconf): Briefly explain the
gpgconf.ctl syntax.