builtin's --help should use pager when the output does not fit in the screen

[krobelus]

E.g. complete -h prints a bunch of useful information but I almost always need to scroll up a or use a pager (\ep or \eh in this case). I think it should use a pager by default if the output is a tty and there are more lines than one screenful, just like git diff and others.

Probably also for functions.

[floam]

This is a good idea, and it doesn't need to be implemented in the builtins: __fish_print_help is the fish function invoked for this and it's easy to improve.