We were making a copy of the fifo name and stripping any domain name,
but then not using the value.

Reduce number of local variables where possible; consolidate lines.

This marker value is only used in one place.

If getting the DB handle failed during the load or reload of the
configuration, we would leak the XML structure we just allocated for
the configuration.

The goal here is to describe theory.  We want to illuminate what the
code is trying to do and what the core concepts are.

If FS is not behind NAT, then every call generates at least three
INFO-level log messages:

  [INFO] switch_nat.c:589 NAT port mapping disabled

This is useless noise.  The message is only interesting if you do have
NAT enabled but mapping disabled, which might indicate a configuration
issue.

With this change, we just skip the entire nat_add_mapping function if
the NAT system isn't initialized or we're not behind NAT.