Eliminates a number of allocations, because callers don't need to copy
the strings to a newly allocated buffer only to null-terminate them.
And most callers don't need to have a null-terminated string.

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

This code (added 7 years ago with commit b233c145) has been obsoleted
by the SongLoader class (added 3 years ago).

Eliminate global variables.

The "tagtypes" command now has several sub commands which can be used
to edit that mask.

handle_tagtypes() is also being moved to ClientCommands.cxx, because
that command will be extended to access client-specific settings.

This compatibility hack was accidently broken by commit f072cbbb

Avoid duplicate AllocatedPath::FromUTF8() invocations in two callers.

Wrapper for protocol/ArgParser.cxx.