Added replacement functions sys_popen and sys_pclose. These are based

Added replacement functions sys_popen and sys_pclose. These are based
on the glibc source code and are safer than the traditional popen as
they don't use a shell to exec the requested command. Now we have
these functions they can be tightened up (environment etc.) as required
to make a safe popen. It should now be safe to add the environement
variable loading code to loadparm.c
Jeremy.
(This used to be commit b52e92b09d4ca3b66e534f520468dee27065d048)