lua: Add wrapper for system posix module

Add a wrapper for protected loading of the "real" posix Lua
module, so we can trap errors and generate a user friendly
error message.
lua: update users of posix module to reference flux-lua.posix

Update front-end commands and libraries that use the posix module
to use the flux-lua.posix wrapper instead. This will change the
generic lua backtrace error to a simpler, user friendly one.

Only front-end commands and modules are updated. Backend stuff
won't help users anyway, and likely won't be invoked if front-end
commands fail.
flux-wreckrun: signal handler update for newer posix module

Handle both old style signal-as-a-table and new style signal-as-a-function
in wreckrun. Uses runtime detection to determin if posix.signal
is a table or a function.

Fixes #5
lua: Fix make check in src/bindings/lua

Fix 'make check' for lua bindings, and ensure necessary sources
and scripts are distributed in a 'make dist' tarball.
Merge pull request #19 from grondo/luaposix-fixes

lua-posix fixes