For strrchr().  Fixes build failure.

These are the default values already.

Replaces its own weird buffering code.

Code simplification.

Add enum InputResult which is a tri-state.  Input plugins may now fail
and just become unavailable.

libsmbclient is not thread-safe nor reentrant.  We must protect all
function calls with a global mutex, unfortunately.

Merge duplicate code.

Don't duplicate the MIME type when it gets passed to a std::string.

The name "spt" didn't make much sense for this plugin.

[mk: modified to use MultiSocketMonitor instead of SocketMonitor]

Users now have to call Close() explicitly.  This simplifies using the
class, as most users have automatic socket management already, and
Steal() had to be used often.