PROXY: implemented native proxy support

This patch creates a hook into the net_connect*() methods which call a
method to connect to a proxy.

Previous solution to send certain strings in the normal IRC dialog was
some kind of hack as most proxies require some kind of negotation.

E.g. HTTP proxies sent a 'HTTP/1.0 200 Connection established' HTTP header
and clients have to wait for it.  Else, sent bytes of the following IRC
login will be dropped silently.

With old method, it is also impossible to tunnel SSL IRC connections
through the proxy as proxy speaks plain text or a special protocol while
e.g. 'CONNECT ... HTTP/1.0' will be encrypted with key of IRC server.

There are further enhancements possible: the whole net_connect stuff
should be made asynchronously. Currently, only the hostname is resolved
in the background (which makes little sense of local proxies usually).
PROXY/SIMPLE: added simple proxy method

This method implements the string + string_after mechanism implemented by
previous irssi versions.

To use, set
* proxy_type to 'simple' or keep it empty
* string + string_after in the known ways
PROXY/HTTP: added methods for HTTP proxies

This patch adds code for connecting through HTTP proxies. Open issues are:

* support of proxy authentication
* a possible DOS due to the usage of g_io_channel_read_line_string() which
  does not allow to specify a maximum length of line.

To use this method:
* set 'proxy_type' to 'http'
PROXY/SOCKS5: added methods for SOCKS5 proxies

This patch adds code for connecting through SOCKS5 proxies. It was
primarily written for use with TOR, so there are some open issues:

* it only allows to make proxy requests with full hostnames; ipv4/ipv6 is
  not supported

* GSSAPI authentication (which is mentioned as mandatory in RFC 1928) is
  not implemented

* plaintext authentication is untested

To use it
* set 'proxy_type' to 'socks5'
PROXY: merge proxy methods into buildsystem

This patch adds the code and rules to build the various proxy methods.
http-proxy: fixed bad return value
Porting some stuff to modern git version
no patching unrelated stuff