Remove another dependency on the config/Global library.

Closes #296

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Drop support for misencoded tags.  People should not be using ID3v1
anyway.

The old struct config_param remains only for top-level string options.

sed gone horribly wrong.  D'oh!

This commit adds the NeighborPlugin API which can be used to detect
nearby file servers that can be used by input plugins.  This list of
servers is exported using the new "listneighbors" command.  The idle
even "neighbor" notifies interested clients when a new neighbor is
found or an existing one is lost.

There's a lot missing currently: protocol&user documentation, and a
way to "mount" remote servers into the music database.  Obviously,
some code from the UPnP database plugin can be moved to a neighbor
plugin.

Look up top-level config options by enum (= integer), not by name
string.