bbf99143's test called syscall.Getsid, which Go's syscall package has on
macOS and the BSDs but not on Linux: go vet ./... (and every test run) on
Linux stopped at "undefined: syscall.Getsid", so the ubuntu Test job failed
at Vet on main and on every PR. golang.org/x/sys/unix, already required,
has Getsid on every Unix.
Verified: GOOS=linux, darwin, freebsd and android go vet ./internal/proc;
go test -run TestAskShellOwnSession ./internal/proc on macOS. Linux is run
by this PR's ubuntu job; there is no Linux machine here.