build-infra: hgforest.sh cannot be interrupted safely.

When "sh get_source.sh" is used fetch the sources, it cannot be safely interrupted.
Even though mercurial has support for proper rollback on ctrl-c, the ctrl-c does not
reach the mercurial processes because of the way hgforest.sh spawns the background
mercurial processes. This leaves the mercurial repositories in unsafe states, somtimes
locked sometimes broken. This is especially noticeable when cloning from locations
where the network is not fast.

The fix consists of tracking the background processes and sending -TERM to them,
when the user presses ctrl-c.