configure.ac: Make sure that logind is enabled if requested, make --enable-logind...

configure.ac: Make sure that logind is enabled if requested, make --enable-logind default

Before this commit, if configured with --enable-logind, but libsystemd
is not found, configure silently succeed, however logind is efficiently
disabled.
With this commit, the configure fails if logind is not explicitly
disabled and libsystemd is not found.
--disable-logind is mandatory if logind integration should not be used.

Automatic detection is disabled by Alejandro Colomar's request.
Extra help in the error message is added by lslebodn's request.

Signed-off-by: Evgeny Grin (Karlson2k) <k2k@drgrin.dev>