It is now set to 0 when progs are loaded and every time
PR_ExecuteProgram() returns. This takes care of the default case, but
when setting parameters, pr_argc needs to be set correctly in case a
vararg function is called.
This makes gib depend on gamecode, but removes the dependency on gib from
ruamoko. Unfortunately, carne now needs to be linked against gamecode even
though it never uses it.