[Chicken-hackers] Additions to posixwin.scm

[Kon Lovett]

Hi,

I added signal-handler & set-signal-handler! to posixwin, along w/  
the signal/* definitions that seem to be available. Also got rid of  
spawnvp/execvp warning.

Only tested w/ MinGW. Need VC & Cygwin.

Thank you,
Kon