posix: add missing locals before pclose and popen_pipeline.
posix: add missing locals before pclose and popen_pipeline.

* lib/posix.lua.in (pclose, popen_pipeline): Make sure to import
API module table entries into locals for easier refactoring if
these functions are moved in a future revision.

ardje/luaposix,rrthomas/luaposix,snegovick/luaposix,rrthomas/luaposix,ardje/luaposix,snegovick/luaposix,luaposix/luaposix,jimmyharris/luaposix,jimmyharris/luaposix