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, gnupg_cachedir, dirmngr_socket_name) [W32]: Ditto.
* common/logging.h (JNLIB_LOG_NO_REGISTRY): New.
* common/logging.c (no_registry): New variable.
(log_set_prefix, log_get_prefix): Set/get that variable.
(do_logv): Do not check the registry if that variable is set.

Beware: This code has not been tested because it is not yet possible
to build GnuPG 2.1 for Windows. However, the code will be the base
for an implementation in 2.0.

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 is empty or consists
only of empty or '#' comment lines.

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