httpd: treat errors from stdin correctly.

httpd: treat errors from stdin correctly.

Fron bug report:

If a CGI or proxied connection is rudely aborted (SIG_{KILL,BUS,SEGV})
then httpd will spin madly the poll loop in:

networking/httpd.c:1080
cgi_io_loop_and_exit()

Upon investigation I found that pfd[0].revents == 0x0018 (POLLHUP|POLLERR),
which leads to empty read, but the pfd[0].fd (STDIN_FILENO) is left open,
and in the FD list given to poll() which immediately returns to once
again inform the loop of (POLLHUP|POLLERR) condition of pfd[0].fd.
This continues until pfd[FROM_CGI].revents != 0

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>