syscall: restore original NOFILE rlimit in child process

syscall: restore original NOFILE rlimit in child process

If we increased the NOFILE rlimit when starting the program,
restore the original rlimit when forking a child process.

For #46279

Change-Id: Ia5d2af9ef435e5932965c15eec2e428d2130d230
Reviewed-on: https://go-review.googlesource.com/c/go/+/476097
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Bypass: Ian Lance Taylor <iant@google.com>