maint: use table.unpack over unpack for 5.3.0 compatibility.

Lua 5.3 has table.unpack but not _G.unpack; Lua 5.2 has
table.unpack and _G.unpack; Lua 5.1 has _G.unpack but not
table.unpack
* lib/posix.lua.in (unpack): Use table.unpack over unpack, to
work on all three!
* lib/posix/deprecated.lua (unpack): Likewise.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>