syscall: work around FreeBSD execve kernel bug
syscall: work around FreeBSD execve kernel bug

FreeBSD's execve implementation has an integer underflow in a bounds test which
causes it to erroneously think the argument list is too long when argv[0] is
longer than interpreter + path.

R=rsc, bradfitz, rsc1
CC=golang-dev
http://codereview.appspot.com/4259056

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go