Moved nested #includes out of osrf_prefork.h and into the implementation files.

Moved nested #includes out of osrf_prefork.h and into the implementation files.
Somne of the #includes turned out to be unnecessary, so I eliminated them.

In osrf_prefork.c: instead of dynamically allocating a prefork_simple,
allocate it on the stack.  That way we avoid a malloc() and free().

Renamed prefork_free() to prefork_clear(), since it no longer frees the
prefork_simple itself, but only memory owned by the prefork_simple.  Also
it now returns void instead of int, since there are no errors to detect
or report.

Added some comments.

M    include/opensrf/osrf_prefork.h
M    src/libopensrf/osrf_system.c
M    src/libopensrf/osrf_prefork.c

git-svn-id: svn://svn.open-ils.org/OpenSRF/trunk@1885 9efc2488-bf62-4759-914b-345cdb29e865