release v0.21.8

Closes https://github.com/MusicPlayerDaemon/MPD/issues/526

This enables the input buffer for remote files and caches file
contents in MPD.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/376

Commit b3a45833 added a LocateUri()
call to several playlist commands, which applied InputPlugin URI
scheme verification to playlist URIs.  This broke the SoundCloud
playlist plugin which uses "soundcloud://" URIs for which no input
plugin exists.

This commit allows the caller to specify the kind of plugin which
shall be used to verify the URI.  Right now, only "input" is
implemented; "storage" uses the "input" verification for now; and
"playlist" has no verification at all (for now).

Closes https://github.com/MusicPlayerDaemon/MPD/issues/528

There is already one call in ReadServers(), which is the correct place
to do it.

Fixes Travis failure with Meson 0.50:

 ERROR: Could not detect Ninja v1.5 or newer

Split several printf() calls to make it easier to deal with all those
#ifdefs.