configure.ac: prepare for 0.19.1
util/DynamicFifoBuffer: make the "Range" type public

Export it from the protected base class.  This fixes a build failure
on Mac OS X.
NEWS: fix typo in version number
NEWS: add missing line
ThreadInputStream: add assertions
ThreadInputStream: call ThreadRead() inside the thread instead of Read()

Fixes deadlock bug in the "mms" plugin.
Makefile.am: distribute systemd/mpd.socket

The file systemd/mpd.service.in is being distributed implicitly, but
systemd/mpd.socket is not and needs to be added to EXTRA_DIST.