disowntty (which must be run as "exec disowntty") relinquishes ownership of the
tty or pty on which it is run. This then allows gdb to run the debugged process
with this device as its controlling tty, via the 'set inferior-tty' command.

disowntty is only needed under Linux. Under Cygwin, 'set inferior-tty' works
fine without it. gdb in openbsd does not have that command.

disowntty.c was first published by Jan Kratochvil in
https://sourceware.org/bugzilla/show_bug.cgi?id=11403
