properly escape the introduction text

This was suggested by Chris Down
use dash-compatible source command '.'

This was suggested by Chris Down
Replace [ -n "`which ...`" ] tests by `type ... >/dev/null 2>&1`

This was also pointed out by Chris Down - Thank you.
change default editor to "xterm -e vi".

gvim probably assumes to much. This was
proposed by Petr Šabata. Thank you.