core: Avoid explicit locks in the debug code.

* src/debug.c (debug_lock): Remove.  Also remove all users.
(_gpgme_debug): Use gpgrt_bsprintf to prepare the output and finally
print using standard fprintf.  Reformat to prefix to be narrower.

Note that the locks are now implicitly done using the systems stdio.

The threadid is now printed with 4 digits in hex and thus without the
angle brackets and the 0x. However it is still a hex number even if it
may look like an octal number. The hex letters are uppercase to make
searching in locks easier iff the threadid happens to have a letter in
it.

• Signed-off-by: Werner Koch <wk@gnupg.org>