Made more.lua command accept more keys

Added more keys for the command to recognise ("page down" goes to the next page in the same way as "space", "enter" and "down" print just the next line)
Moved the code to print lines from the file to a function to reduce the space used by repeating it for the new keys
One question right away, though, the return value is used in a boolean check on line 48.
Are you aware that `not not 0 == true` in Lua? I.e. that zero is *not* false as it is in, say, C?