unix: add Signalfd on linux

Also add the SFD_* constants and SignalfdSiginfo type.

Change-Id: I3415c212b85c9d5a940ba2bf70f43739a2630d78
Reviewed-on: https://go-review.googlesource.com/c/157897
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_linux.go

@@ -1467,6 +1467,7 @@ func Setgid(uid int) (err error) {
 
 //sys	Setpriority(which int, who int, prio int) (err error)
 //sys	Setxattr(path string, attr string, data []byte, flags int) (err error)
+//sys	Signalfd(fd int, mask *Sigset_t, flags int) = SYS_SIGNALFD4
 //sys	Statx(dirfd int, path string, flags int, mask int, stat *Statx_t) (err error)
 //sys	Sync()
 //sys	Syncfs(fd int) (err error)
@@ -1683,7 +1684,6 @@ func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 // Shmdt
 // Shmget
 // Sigaltstack
-// Signalfd
 // Swapoff
 // Swapon
 // Sysfs