pty functionality in libutil.a

[Joe Buehler]

There are several pty functions in libutil.a, apparently provided by
the inetutils package, that could profitably be moved
into cygwin proper.

It looks as though they are standard BSD pty functions, and are very
small.

The performance of one of them, forkpty(), could be improved by
using vfork() instead of fork(), if the setsid()/vfork()
interaction gets fixed.

Joe Buehler