Retry open_cloexec for signals other than SIGINT - #10251

This retries {w,}open_cloexec if it gets EINTR, unless it got a cancel signal (i.e. SIGINT).

That means if you do cat >fifo (where "fifo" is a fifo without a reader), you can only stop it if you ctrl-c, not if you e.g. resize the terminal.

It also no longer prints the warning when you press ctrl-c - SIGINT is expected, we don't print anything in a lot of cases.

This affects all uses of open_cloexec, e.g. in cd. Most probably won't see a difference, the man page for open(2) on my system claims:

While blocked waiting to complete an open of a slow device (e.g., a FIFO; see fifo(7))

Fixes #10250

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst