When fish runs with job control enabled, it transfers ownership of the
tty to a child process, and then reclaims the tty after the process
exits. If job control is disabled then fish does not transfer or reclaim
the tty.
It may happen that the child process creates a pgroup and then transfers
the tty to it. In that case fish will not attempt to reclaim the tty, as
fish did not transfer it. Then when fish reads from stdin it will
receive SIGTTIN instead of data.
Fix this by unconditionally claiming the tty in readline().
Fixes#9181
This changes how fish attempts to protect itself from calling tcsetpgrp() too
aggressively. Recall that tcsetpgrp() will "force" itself, if SIGTTOU is
ignored (which it is in fish when job control is enabled).
Prior to this fix, we avoided SIGTTINs by only transferring the tty ownership
if fish was already the owner. This dated from a time before we had really
nailed down how pgroups should be assigned. Now we more deliberately assign a
job's pgroup so we don't need this conservative check.
However we still need logic to avoid transferring the tty if fish is not the
owner. The bad case is when job control is enabled while fish is running in the
background - here fish would transfer the tty and "steal" from the foreground
process.
So retain the checks of the current tty owner but migrate them to the point of
calling tcsetpgrp() itself.
