unshare: add --forward-signals option to argument parser

unshare: add --forward-signals option to argument parser

Add a new --forward-signals command-line option that will allow
unshare to forward SIGTERM and SIGINT signals from the parent
process to the forked child process.

This commit adds the option parsing infrastructure but does not
implement the signal forwarding logic yet. The flag defaults to 0
(disabled) to maintain backward compatibility.

Signed-off-by: Kiran Rangoon <kiranrangoon0@gmail.com>