Add support for SIGTSTP

SIGTSTP is the signal received when the user sends Ctrl-Z from the
terminal. It is different from SIGSTOP, and was not yet supported by
lua-signal. This pull-requests adds it.

Example use:
```lua
require 'signal'
require 'sys' -- for sleep

signal.signal('SIGTSTP', function() print('HELLO WORLD!') end)

while true do
    print('...')
    sys.sleep(1)
end
```