bug#19235: make-fresh-user-module procedure leaks memory

[Chris Vine]

As far as I can tell the make-fresh-user-module procedure is not called
by guile itself, and providing a global mutex for it with a binding
enabling it to be called from scheme code seems to work fine.

This also makes it straightforward to incorporate in a thread-safe
way the code you suggested to free stale user modules.  However, as I
mentioned, I am a bit reluctant to incorporate code which might break
in the future.  Is there any possibility that a "delete-module!"
procedure could be included within the public guile API for the next
release of guile?  It seems like something that could be useful to
anyone using non-default user modules in their code.

Chris