Housekeeping

setgitproxy is now setsshproxy and unsetgitproxy is now unsetsshproxy
Simplify configuring ssh proxy settings

Uses ProxyCommand with variables in ~/.ssh/config to set proxy. This
removes the need to have the 2 ssh config files: ssh-config-proxy and
ssh-config-no-proxy.

* use setsshproxy <host> <port> to configure ssh to use a proxy
* use unsetsshproxy to configure ssh to stop using a proxy
* unsetsshproxy removes first line of .ssh/config when removing proxy config
which may damage ssh config if setsshproxy was not run before
Update README on new ssh proxy procedure