SSH server support

[totaam]

Using paramiko (#1646), add a bind-ssh option and automatic upgrades of TCP sockets to SSH.

[totaam]

• r19950: main code merged
• r19951: basic support for password authentication (works with sys, reject, allow, sqlite, etc)
• r19952 public key authentication
• r19954 support older clients that try to execute a complicated shell command
• r20014 + r20015 + r20017: win32 server support, load ssh host keys from platform specific locations (ie: support standard locations on win32: C:\ProgramData\ssh and C:\Windows\system32\OpenSSH)

Still TODO:

• win32: generate host keys if we can't find any during installation?
• try macos servers (add platform paths for keys?)
• support more subcommands when ran from the proxy? "_proxy_start" etc?
• update wiki

[totaam]

Updates:

• SSH and Network wiki pages have been updated
• r20134 add system key locations for macos
• r20049 + r20139: bundle openssh so we can generate a host key during install, not sure why we can't use puttygen to convert putty ppk to openssh format: this is meant to work but pops up the GUI every time I tried (and I've tried with puttygen from upstream and also the one from MSYS2):

$ puttygen id_dsa.ppk -O private-openssh -o id_dsa
$ puttygen id_dsa.ppk -O public-openssh -o id_dsa.pub

Anyway, with this in place the win32 server will support ssh connections out of the box.
We don't generate host keys for the macos server. (the system has ssh host keys installed in /private/etc, and we do support this path, but those keyfiles aren't readable by a non-root user..)

@maxmylyn: you can now connect to your xpra servers using the SSH transport, on the same port as TCP connections, or to other ports when using bind-ssh=.