[PATCH] callable argument for PTY.getpty (aka PTY.spawn)

Hi!

The patch attached enables you to pass a callable object to PTY.spawn,
so that if you do so, of course, the object will be called (instead of
exec'ing an external program).

Rationale: sometimes you want to do some initialization on the pty
before starting the external program, or maybe you don't even want to
call an external program...

Example: this whole thing popped up when I was writing a small readline
wrapper. The pty spawned for the util to be wrapped is to be put into
non-echo mode prior to starting the util. With the current pty the only
way to do this is as follows:

 inp,out,cpid = 
  PTY.spawn(*(["ruby", "-e", 'system "stty -echo"; exec *$*'] + $*))

With this patch I can do just

 inp,out,cpid = PTY.spawn(proc { system "stty -echo"; exec *$*})

Csaba