[PATCH] mutex-unlock! should allowed #f as timeout according to srfi-18

[Nala Ginrut]

According to srfi-18:

---------------------cut----------------------
Time objects and timeouts

......

a time object represents an absolute point in time
an exact or inexact real number represents a relative time in seconds
from the moment the primitive was called
#f means that there is no timeout
---------------------end----------------------

Attached the patch to fix such a situation:
==========
scheme@(guile-user)> (mutex-unlock! m condv #f)
ERROR: In procedure unlock-mutex:
ERROR: Wrong type (expecting real number): #f
==========

And I do know that `mutex-unlock!' uses timeout as the last optional
argument, usually users won't pass timeout when they don't need it.
But I do think it's necessary to stick to the standard as possible as we
can.

Thanks!

0001-mutex-unlock-should-allowed-f-as-timeout-according-t.patch
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