net/http/cgi: Correctly pass down the REMOTE_PORT value for CGI requests.

Currently when we get a CGI or FCGI request, the remote port of the client
is hard coded to zero, despite nearly every webserver passing down the
REMOTE_PORT variable.

This was likely originally excluded because the CGI RFC (rfc3875) does not
mention anything about the remote port of the client. However every webserver
tested does pass REMOTE_PORT down. This includes Apache 2.2, Apache 2.4,
nginx and lighttpd.

Fixes #8351

Change-Id: I4c6366cb39f0ccc05e038bd31d85f93b76e8d0c8
Reviewed-on: https://go-review.googlesource.com/1750
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
runtime: run libc SIGSETXID and SIGCANCEL handlers on signal stack

These signals are used by glibc to broadcast setuid/setgid to all
threads and to send pthread cancellations.  Unlike other signals, the
Go runtime does not intercept these because they must invoke the libc
handlers (see issues #3871 and #6997).  However, because 1) these
signals may be issued asynchronously by a thread running C code to
another thread running Go code and 2) glibc does not set SA_ONSTACK
for its handlers, glibc's signal handler may be run on a Go stack.
Signal frames range from 1.5K on amd64 to many kilobytes on ppc64, so
this may overflow the Go stack and corrupt heap (or other stack) data.

Fix this by ensuring that these signal handlers have the SA_ONSTACK
flag (but not otherwise taking over the handler).

This has been a problem since Go 1.1, but it's likely that people
haven't encountered it because it only affects setuid/setgid and
pthread_cancel.

Fixes #9600.

Change-Id: I6cf5f5c2d3aa48998d632f61f1ddc2778dcfd300
Reviewed-on: https://go-review.googlesource.com/1887
Reviewed-by: Ian Lance Taylor <iant@golang.org>