Merge branch 't/versioning_improvements'

Merge branch 't/versioning_improvements'

Cooking for ~2 weeks.

	15e99a version.c: Mark version_git() as const.
	9bdebf Remove CODENAME macro.
	be2f6b gui: Use version_single_line().
	d60dae Improve man page layout.
	5dbc9a afh/play: Include supported audio formats in help output.
	042767 Use self-made help to avoid recompilations on version changes.
	48f1fc Provide "purpose" texts.
	aa74a9 Revamp ggo help.
	9f7a49 afh_recv: Replace ggo text section by description.
	b59e0e Make gengetopt descriptions work.
	06b3e7 Introduce version.c to limit recompilation on version changes.
	75feac Make all commands print git version and improve version string.
	b01605 Avoid unwanted log messages during startup.
	625fdb Don't check return value of command line parsers unnecessarily.
	533b03 Build receivers, filters and writers without -h and -V support.
	b59a3c filter: Wrap lines in the available filter list.
	9e56d3 audioc: Print config file errors.
	e5264d doc: Rewrite udp sender description
	5ec373 client: Fix typo in comment.
	6d5159 client: Remove duplicate include.