swapon: improve man page notes about holes

Signed-off-by: Karel Zak <kzak@redhat.com>
include/debug: improve and cleanup

* use ul_debug_ prefix for all routines
* support <NAME>_DEBUG=all also for programs without debug mask names
  (so we can avoid 0xffff mask in man pages)
* add function to print debug help

Signed-off-by: Karel Zak <kzak@redhat.com>
libblkid: clean up debug, add "help" debug mask

Signed-off-by: Karel Zak <kzak@redhat.com>
libmount: clean up debug, add help debug mask

Signed-off-by: Karel Zak <kzak@redhat.com>
libsmartcols: clean up debug, add help debug mask

Signed-off-by: Karel Zak <kzak@redhat.com>
libfdisk: clean up debug, add help debug mask

Signed-off-by: Karel Zak <kzak@redhat.com>
docs: update info about env debug variables

Signed-off-by: Karel Zak <kzak@redhat.com>