The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes false negatives:

 http://foo/dav/example.ogg mismatches http://foo/dav/

.. because StringAfterPrefix() returns just "example.ogg", without
trailing slash (it existed, but was eaten already by the base
matcher).

This doesn't make sense on Android; there is no home directory for
users.

Prepare to add more modes.