getopt: ensure we find _G.arg from Specl nested setfenv environments.  (issue #27)

Although only necessary for Lua 5.1, this fix is harmless for Lua
5.2, and we support both!
* src/getopt.lua (processArgs): Use only fully qualified _G.arg
references, for Lua 5.1 Specl compatibility.
Fixes issue #27.