This disables job control inside command substitutions. Prior to this
change, a cmdsub might get its own process group. This caused it to fail
to cancel loops properly. For example:
while true ; echo (sleep 5) ; end
could not be control-C cancelled, because the signal would go to sleep,
and so the loop would continue on. The simplest way to fix this is to
match other shells and not use job control in cmdsubs.
Related is #1362
sigint2 would hang (probably because of different semantics in signal
delivery?)
wcstod isn't implemented correctly, so math can't do hex numbers.
OpenBSD only passes the filename as argv[0] and doesn't give us another feature I know of, so status fish-path can't work.
