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 config_parse_path().

default_value defaults to nullptr.

Drop support for misencoded tags.  People should not be using ID3v1
anyway.

The old struct config_param remains only for top-level string options.

Replaces the dirty code in config/ConfigFile.cxx.

Move code from config_global_finish().