(locate-library): Use locate-file(-completion).

(locate-library): Use locate-file(-completion).
(help-split-fundoc): Tighten the regexp and recognize a usage-info
where the function name is `fn'.
(help-function-arglist): Follow aliases and macros.
(describe-function-1): Use keymapp.
If the docstring contains a usage info, use it even if the function is
not a subroutine.  Find usage info in aliases' docstring if necessary.
(describe-variable): Get the value before setting up the *Help* buffer.