passwordfd: read password during option processing (#420)

passwordfd: read password during option processing (#420)

When configured to detach from the controlling terminal, pppd closes
file descriptors 0, 1, and 2 before the passwd hook is called. If the
user passes 0, 1, or 2 to the passwordfd option, pppd will fail to read
the password.

To work around this, treat passwordfd as a special option and read the
password during option processing, before pppd closes it.

Bug: https://bugs.gentoo.org/209294

Signed-off-by: Mike Gilbert <floppym@gentoo.org>