x/sys/unix: add Getresuid and Getresgid

[mateusz834]

The x/sys/unix package can set the suid (saved UID) through setresuid, but there is no way to retrieve it (the getresuid(2) is missing).

I propose adding Getresuid to the x/sys/unix package (also probably Getresgid) with following signature:

func Getresuid(ruid *int, euid *int, suid *int) (err error)
func Getresgid(rgid *int, egid *int, sgid *int) (err error)

[seankhliao]

Feel free to just file a CL, missing things for x/sys/unix can just be added without needing to go through proposal.

[gopherbot]

Change https://go.dev/cl/499055 mentions this issue: unix: add Getresuid, Getresgid on linux