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.

New in clang 3.6.

Enables using range-based "for".

Error message was 'too few arguments for "find"' because the "base"
constraint was not supported, and no other constraints remained.

Temporary hack for the experimental no-GLib build.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

Fixes clang warning.

Fixes clang warning.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

Restores the features from the previous draft commands "findin" /
"searchin".

According to the protocol documentation, matching the file name was
wrong.  This removes some awkward special-case code.

Replaces GLib's g_ascii_strcasecmp().