Exit successfully with --help

Passing --help is a legitimate use, so it should not exit(1). Besides,
this is useful to test whether the command is available.
Clean up compile warnings and default to -Wall.

Hopefully this will make introducing new bugs more obvious.
Allow pipe output to run even if the reading end is not open (yet)
mdns_external: don't warn loudly, these are often expected to fail

eg. on many avahi and all tinysvcmdns systems.
Requesting progress and metadata in MDNS announcement.
Switching to sub handlers in handle_set_parameter by content-type.
Moved actual parameter handler.
Added empty handlers for cover art.
Added preliminary parsing code for DMAP tags.
Added metadata structures and methods.
Added command line option to set the cover art output directory.
Added player methods for printing metadata and setting the artwork image.
Added metadata.c to makefile.
Reading the progress: parameter to prevent warnings.
Settings response status in main handler.
Writing the cover image to a file.
This only happens if the cover directory option is set.
Using the MD5 hash with a prefix as the file name. Don't overwrite the
file if it already exists.
Calls to the player for setting and clearing the album cover.
Supported types are png and jpeg. Setting the file extension.
Removed comments. Naming...
Setting the metadata from parsed data and passing it to the player.
Cleaned logging for SET_PARAMETER requests.