config: add modulifile option to customize moduli file location

Currently all files can be customized via sshd_config, however, the
moduli file cannot.

Running sshd in unprivileged context requires customization of all
resources, especially when some distributions sets the moduli as world
unreadable.

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>
Bug-Url: https://bugzilla.mindrot.org/show_bug.cgi?id=2277