Generate correct xfree86 keymaps when evdev is used

Use a mapping table to map xfree86 keycodes to evdev keycodes and obtain
consistent keysyms for keymaps. This way, it is now possible to generate
correct keymaps when evdev driver is used on host.
configure: Correct the use of AC_ARG_ENABLE() macro

Fix the use of AC_ARG_ENABLE() so that --disable-foo configure options
are respected.

The third and fourth arguments of AC_ARG_ENABLE are respectively
ACTION-IF-GIVEN and ACTION-IF-NOT-GIVEN. The former is executed if
either --enable-foo or --disable-foo is specified on command line, the
latter if neither is. Additionally, the specified value is stored in
$enable_foo variable.

Therefore, the common practice is to provide null ACTION-IF-GIVEN
(and use the value of $enable_foo), and to set a default for $enable_foo
in ACTION-IF-NOT-GIVEN. The commit does that, and adjusts the further
conditionals to test $enable_foo properly.
configure: Add --disable-pam and deprecate --enable-nopam

Use simple logic for PAM control to avoid confusion and double
negations. Keep --enable-nopam deprecated for backwards compatibility.
configure: Ensure PKG_CONFIG before querying systemd unit path

Use PKG_PROG_PKG_CONFIG to ensure that $PKG_CONFIG is set before it is
used to query the systemd unit install path.
Merge pull request #181 from mgorny/configure-fixes

Configure fixes
sesman: check for null from crypt()
Merge pull request #180 from mgorny/evdev-genkeys

Generate correct xfree86 keymaps when evdev is used