…ignal.

This moves the SSL loading code from MetaParams::read() into a separate
method, loadSSLSettings(). It also hooks up the SIGUSR1 signal handler
to reload SSL settings and apply the setings to each suitable virtual
server.

A follow-up commit will change MetaParams::read() to also use loadSSLSettings(),
however it was left out of this commit to aid in reviewability.

…tings().

This changes MetaParams to use the newly-introduced loadSSLSettings() method
to load SSL settings.

This changes Server::initializeCert() to always reset the
Server's SSL state before reloading it.

Previously, we didn't reset the state.

Not doing so *does* actually works for the case where we
load the certificate and key from the per-vserver configuration
in ServerDB.

However, it doesn't work when reading from Meta, because
the old Server::initializeCert() would only use the settings
from Meta if qscCert/qskKey were null.