This function will set a shared lock on the directory that the
current program was loaded from. This can be accessed through
the path PROGDIR:. The use of this path is to allow the program
to easily access files which are supplied with the program.
INPUTS
lock
The lock to set as the new program directory. NULL
is a valid value.
RESULT
This function will return the old program directory lock.
NOTES
This function will not duplicate the lock, so you should not
free the lock.