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
https://golang.org/cl/4259056

src/pkg/syscall/exec_unix.go

@@ -238,6 +238,10 @@ func forkExec(argv0 string, argv []string, envv []string, traceme bool, dir stri
 		dirp = StringBytePtr(dir)
 	}
 
+	if OS == "freebsd" && len(argv[0]) > len(argv0) {
+		argvp[0] = argv0p
+	}
+
 	// Acquire the fork lock so that no other threads
 	// create new fds that are not yet close-on-exec
 	// before we fork.