runtime: automatically bump RLIMIT_NOFILE on Unix [1.20 backport]

[gopherbot]

@ianlancetaylor requested issue #46279 to be considered for backport to the next 1.20 minor release.

@gopherbot Please open backport issues.

CL 393354, which first appeared in Go one point nineteen, causes Go programs that start other non-Go programs to lose the soft NOFILES rlimit. This breaks programs that care about that. CLs 476096 and 476097 fix the problem by changing the syscall fork and exec functions to restore the original soft rlimit if appropriate. This is a request to backport those changes to the currently supported releases.

[heschi]

Fix CL seems largeish but harmless; as long as the backport CLs aren't more complicated, approved.

[gopherbot]

Change https://go.dev/cl/478659 mentions this issue: [release-branch.go1.20] syscall: restore original NOFILE rlimit in child process