common: Use a common gpgconf.ctl parser for Unix and Windows.

* common/homedir.c (gpgconf_ctl): new struct.
(string_is_true): New.
(parse_gpgconf_ctl): New.  Based on the former code in unix_rootdir.
(check_portable_app): Use parse_gpgconf_ctl and the new struct.
(unix_rootdir): Ditto.

This is a unification of the gpgconf.ctl mechanism. For backward
compatibility we need to keep the empty (or actually only comments)
method as used formerly under Windows. Iff one really wants a
portable application the new portable keyword should be used, though.

Noet that the Windows portable stuff has not been tested for quite
some time.