Make LogError()/FormatError() wrappers for those.  Now we can log
exceptions with a lower level.

Prepare for templated functions.

Map LogLevel::INFO to G_LOG_LEVEL_INFO, and LogLevel::DEFAULT to
G_LOG_LEVEL_MESSAGE.  Now client connect/disconnect message are only
logged on log_level "secure".

Copy the according file from another project (i.e. XCSoar).  This will
allow copying more code more easily.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Don't free the string right after calling log_init_file().  Add a new
function log_deinit() that frees the string on shutdown.

This fixes cycling the log file after SIGHUP (Mantis ticket 0003524).

Use GLib the logging functions g_debug(), g_error() instead.

This updates the copyright header to all be the same, which is
pretty much an update of where to mail request for a copy of the GPL
and the years of the MPD project. This also puts all committers under
'The Music Player Project' umbrella. These entries should go
individually in the AUTHORS file, for consistancy.

Removed GLib logging wrappers which are not used anymore.

The logging library currently has 3 constructor functions: initLog(),
open_log_files(), setup_log_output(), called in this order.  Merged
the first two.

We want to remove gcc.h eventually. This takes care of all the G_GNUC_PRINTF
macros.

We want to remove gcc.h eventually. This takes care of all the
G_GNUC_NORETURN macros.

Use the bool data type for flags.

The logging functions from log.h are deprecated, and the code should
use GLib logging instead.  Make ERROR(), WARNING() etc. call g_logv()
internally.

The warning buffer is a complex piece of code for no good reason.
Remove it and find a better solution, e.g. open the log file earlier.

"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

Include only headers which are really used.