Don't spawn man for --help

Don't spawn man for --help

Convert pkaction and pkttyagent to use GOptionContext.

Don't convert pkcheck and only add --help output text because its
non-standard --details(which requires two arguments) can't be
implemented using GOptionContext.

Don't touch pkexec, in a (futile?) attempt to minimize the amount
of complex code running before authentication.

This leaves the option processing lax as it was (e.g. accepting
contradicting options, ignoring non-option arguments), and should only
affect the handling of --help and behavior when invalid arguments are
detected.

https://bugs.freedesktop.org/show_bug.cgi?id=29936