Forward SIGTERM and SIGINT in wrapper script

[matthijskooijman]

This makes sure that if the wrapper bash script receives a SIGTERM or
SIGINT, that is forwarded to the actual player to terminate it. Then,
the wrapper script does not terminate itself until the player does,
allowing cleanup to happen as normal.

This is helpful when running omxplayer from a script that wants to
terminate the player. It can just terminate the (wrapper) process that
it spawned, which will terminate the script and the player, while still
doing proper cleanup.

Note that SIGKILL cannot be handled in this way, since that kills the
script instantly with no way to trap it.

I've tested this change on Raspbian Buster, running omxplayer f543a0d (porting over just the wrapper script from this git repo).

[bit-bash]

With this PR, I cannot control omxplayer via the keyboard, either natively or via ssh.
I think this is because omxplayer.bin is started in the background.
Not sure of any workaround to both support the signals and allow keyboard control of omxplayer.