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.

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

Merge duplicate code.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.

Look up top-level config options by enum (= integer), not by name
string.

No writers.

config_get_path() was somewhat flawed, because it pretended to be a
function, when it really had a side effect.  The second flaw was that
it did not return the parser error, instead it aborted the whole
process, which is bad style.  The new function returns a duplicated
(modified) string that must be freed by the caller, and returns a
GError on failure.

No caller needs to write.

config_add_block_param() cannot fail, which makes it easier to use.

Used to connect and authenticate to despotify. Provides a singleton
session and a desptoify-track-to-tags function.