Prepare to allow more complex expressions.

Required for "modified-since".

All DetachedSong instances should be converted to LightSong first,
which is a very cheap operation.

Closes #89

Will allow more complex fitler expression, such as negation (#89).

Not yet wired to the protocol.

Consume the ConstBuffer instead of iterating it.  This will allow
adding more complex parsers.

This enforces the "not nullptr" rule.

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Error message sent to client was "basic_string::_M_construct null not
valid" due to passing nullptr to the std::string constructor.

Regression caused by commit 386688b8

Fixes search within mount points, resulting in error "No such
directory".

Closes #190

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.

Move code from SongFilter.cxx.