Add support for block and critical blocks to f90.el.

Add support for block and critical blocks to f90.el.

* lisp/progmodes/f90.el (f90-critical-indent): New option.
(f90-font-lock-keywords-2, f90-blocks-re, f90-end-block-re)
(f90-start-block-re, f90-mode-abbrev-table): Add block, critical.
(f90-mode): Doc fix.
(f90-looking-at-critical, f90-looking-at-end-critical): New funcs.
(f90-no-block-limit, f90-calculate-indent, f90-end-of-block)
(f90-beginning-of-block, f90-next-block, f90-indent-region)
(f90-match-end): Handle block, critical.