Fix tests for 'scm_c_bind_keyword_arguments'.

* test-suite/standalone/test-scm-c-bind-keyword-arguments.c (error_handler):
  Remove function.
  (unrecognized_keyword_error_handler, invalid_keyword_error_handler,
  odd_length_error_handler): New functions.
  (test_scm_c_bind_keyword_arguments): Use new error handler functions.
Allow #f as timeout argument to unlock-mutex and SRFI-18 mutex-unlock!

Reported by Chaos Eternal <chaoseternal@shlug.org>
Based on a patch by Nala Ginrut <nalaginrut@gmail.com>

* libguile/threads.c (scm_unlock_mutex_timed): If 'timeout' argument
  is false, interpret that as no timeout.

* doc/ref/api-scheduling.texi (Mutexes and Condition Variables):
  Update documentation.