Signed-off-by: Rosen Penev <rosenp@gmail.com>

This way, URI schemes supported by arbitrary storage plugins are
allowed.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/1270

This should translate to ACK_ERROR_ARG instead of ACK_ERROR_UNKNOWN.

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

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330