fix #1956: return controlling terminal to parent

fix #1956: return controlling terminal to parent

The changes introduced in e97f807c388c10250f442b1f16c5315df2ffc2af let the
child in fork_worker take the controlling terminal of the session, without
returning it after finishing.
This breaks using/reading from the terminal after both parent and child exit
- e.g. when the code is called from within a shellscript.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>