do not crash when subshell exits via signal
do not crash when subshell exits via signal

Here is something you can try with a live server installer today: drop
into a subshell and type "kill -9 $$". The installer will crash with an
'Input/output error' from tcsetattr.

What's going on is some deep unix arcana: when the subshell exits via
signal somehow the shell's process group is still in the foreground
(even though there are no processes in it?) and calling tcsetattr() from
a background process is not allowed. So the fix for this is reasonably
simple (or at least: short): call tcsetpgrp() to put our process group
back into the foreground. A background process doing this gets sent
SIGTTOU, but if we ignore that, all is well.

Frankly this is all very strange and I would like a lie down now.

CanonicalLtd/subiquity,CanonicalLtd/subiquity