specl: use loadstring for Lua 5.1 compatibility.

* src/specl.lua (loadstring): If loadstring is not defined, then
this must be Lua 5.2, so define a loadstring work-a-like around
load.
* src/specl.in (slurp): Use loadstring, either from Lua 5.1 core,
or the wrapper above.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: always pass a (small) number to os.exit().

os.exit in Lua 5.1 doesn't accept a boolean, so we pass it 1 if
there are any failures, otherwise 0.
* src/specl.lua (run): Return a small integer suitable for os.exit.
* src/specl.in: Remove completed TODO.

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