As spotted by Brad Fitzpatrick, CL 106838 unexported the value field of
Fsid and Sigset_t, but these types would be useless without any exported
fields. Re-export them via an exception in mkpost.go

Change-Id: I30d3c3b81c8c1b6377f8aa55cad816e3486a4395
Reviewed-on: https://go-review.googlesource.com/107415
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>