w32: Add code to support a portable use of GnuPG.

* common/homedir.c (w32_bin_is_bin, w32_portable_app) [W32]: New.
(check_portable_app) [W32]: New.
(standard_homedir, default_homedir) [W32]: Support the portable flag.
(w32_rootdir, w32_commondir) [W32]: Ditto.
(gnupg_bindir) [W32]: Ditto.

A portable use of GnuPG under Windows means that GnuPG uses a home
directory depending on the location of the actual binary. No registry
variables are considered. The portable mode is enabled if in the
installation directory of the the binary "gpgconf.exe" and a file
"gpgconf.ctl" are found. The latter file shall be empty or consist
only of empty or '#'-style comment lines.

• Signed-off-by: Werner Koch <wk@gnupg.org>