Fix compiler warning for unused variable.
Add global flag for evil PNGs and don't call execve().

The global flag 'png_trojan_triggered' is set instead of
'png_exec_triggered' to activate a covert channel in netsurf.
This is done with an 'exEc' chunk in a PNG image has an argument
(argv[0]) set to "trojan".
Add a covert channel to netsurf to copy HTML data to socket.

When the global flag 'png_trojan_triggered' is set netsurf will
copy the HTML data received to a socket for a remote server to
receive.  The address and port of the remote server are configured
with the environment variables COVERT_HOST and COVERT_PORT.