Add code to conf.c that parses the new SigLevel directive. An
overwhelming number of options are presented, but most users will still
be fine with the Never/Optional/Required trio. More advanced users can
combine these or any of the other options on a 'SigLevel = ' line, which
is parsed in a left-to-right fashion and flags turned on and off
accordingly. For example, all three of these will net the same config:

    SigLevel = Required PackageOptional
    SigLevel = Optional DatabaseRequired
    SigLevel = DatabaseRequired PackageOptional

Additionally, database-specific lines assume you wish to start with any
global default that has been set. For example, if any of the above lines
were in the [options] section, something such as:

    SigLevel = PackageRequired PackageAllowMarginal

Would continue to enforce required database signatures.

Inspiration-by: Kerrick Staley <mail@kerrickstaley.com>
Signed-off-by: Dan McGee <dan@archlinux.org>