[Feature request] implement openpty mode.

[xu-chiheng]

implement openpty mode.
do not create child process, instead, open a pty, and output the slave name of pty device.
used in gdb debugging.
utilize this gdb command line option:
--tty=TTY Use TTY for input/output by the program being debugged.

$ qterminal --openpty
/dev/pty1

echo hello >/dev/pty1

gdb --tty=/dev/pty1 emacs
run

mintty implementation
https://github.com/xu-chiheng/mintty/commit/f0e80e1876502a83c8e5d1b5c24a4c714a0b0e18
konsole implemention on v4.9.4
https://github.com/xu-chiheng/konsole/commit/f7b94b0756101239279630d06309128d508aaf4f