Whitespace changes, trivial commentary changes.

Whitespace changes, trivial commentary changes.
(f90, f90-indent, f90-program-indent, f90-continuation-indent)
(f90-indented-comment-re, f90-imenu-generic-expression)
(f90-mark-subprogram, f90-join-lines): Minor doc changes.
(f90-menu): Shift definition to initialization of f90-mode-map.
Add customization section.
(f90-mode): Minor doc change.  Do not call easy-menu-add.
Set `beginning-of-defun-function' and `end-of-defun-function' to
appropriate F90 functions.
(f90-indent-line, f90-indent-line, f90-indent-subprogram)
(f90-break-line, f90-do-auto-fill, f90-insert-end)
(f90-upcase-keywords, f90-capitalize-keywords)
(f90-downcase-keywords): Change interactive spec.