ssh: handle IPv6 ssh connections

Performing ssh actions towards Kallithea via an IPv6 connection gave the
following error:

$ hg incoming ssh://kallithea@example.com/repo
remote: Traceback (most recent call last):
remote: File ".../bin/kallithea-cli", line 11, in <module>
remote: load_entry_point('Kallithea', 'console_scripts', 'kallithea-cli')()
remote: File ".../python2.7/site-packages/click/core.py", line 764, in __call__
remote: return self.main(*args, **kwargs)
remote: File ".../python2.7/site-packages/click/core.py", line 717, in main
remote: rv = self.invoke(ctx)
remote: File ".../python2.7/site-packages/click/core.py", line 1137, in invoke
remote: return _process_result(sub_ctx.command.invoke(sub_ctx))
remote: File ".../python2.7/site-packages/click/core.py", line 956, in invoke
remote: return ctx.invoke(self.callback, **ctx.params)
remote: File ".../python2.7/site-packages/click/core.py", line 555, in invoke
remote: return callback(*args, **kwargs)
remote: File ".../kallithea/bin/kallithea_cli_base.py", line 79, in runtime_wrapper
remote: return annotated(*args, **kwargs)
remote: File ".../kallithea/bin/kallithea_cli_ssh.py", line 74, in ssh_serve
remote: vcs_handler.serve(user_id, key_id, client_ip)
remote: File ".../kallithea/lib/vcs/backends/ssh.py", line 65, in serve
remote: self.authuser = AuthUser.make(dbuser=dbuser, ip_addr=client_ip)
remote: File ".../kallithea/lib/auth.py", line 407, in make
remote: if not check_ip_access(source_ip=ip_addr, allowed_ips=allowed_ips):
remote: File ".../kallithea/lib/auth.py", line 860, in check_ip_access
remote: if ipaddr.IPAddress(source_ip) in ipaddr.IPNetwork(ip):
remote: File ".../kallithea/lib/ipaddr.py", line 76, in IPAddress
remote:
remote: ValueError: '2' does not appear to be an IPv4 or IPv6 address
abort: no suitable response from remote hg!


This was caused by IPv4-exclusive parsing of the SSH_CONNECTION variable.
With an IPv6 address starting with '2a02:1810:', only the first '2' would
survive.

According to 'man 1 ssh':

SSH_CONNECTION Identifies the client and server ends of the con‐
nection. The variable contains four space-sepa‐
rated values: client IP address, client port num‐
ber, server IP address, and server port number.


So, the client IP address will be the first space-separated word, regardless
of IPv4 or IPv6. Use that knowledge without further parsing.

(commit message by Thomas De Schampheleire)