elx-header-multiline: simplify detection

* Use a single regex which simply matches "continuations lines need to
  have more whitespace in front.
* Drop support for @(#) prefixes used by the 'what' unix command.
  Nobody uses that.
remove elx-header and elx-header-multiline

Use lm-header and lm-header-multiline instead.
These built-in functins have been updated accordingly.
remove elx-with-file use lm-with-file instead

The built-in lm-with-file has been changed to move to the beginning of
the buffer before running BODY.  Use current buffer only if FILE is
nil; no longer check whether it is equal to buffer-file-name.
elx-commentary: replace ^L with empty string
remove elx-package
remove elx-commentary-start and elx-commentary-end

Use the respective lm-* functions instead.  lm-commentary-end will be
updated always return a position before the first non-comment after
the start.
remove elx-homepage

use lm-homepage instead
merge elx-summary-1 into elx-summary; reverse meaning of optional arg
rename elx-keywords{ => -list}

The respective function from lisp-mnt also has the -list suffix.
lm-keywords on the other hand returns a space separated string
of all keywords.
elx-summary: rename argument STANDARDIZE => SANITIZE
elx-remap-keywords: re-indent doc-string
elx-keywords-list: add optional arg SANITIZE

Only remove/replace keywords if it is non-nil.
elx-commentary: add optional arg SANITIZE
elx-keyword-list: return strings like lm-keyword-list
bundle lisp-mnt.el from Emacs-24.3.50

The version of `lisp-mnt' in Emacs-24.3.50 contains some changes which
we depend upon, so that version is bundled with this package in a
directory which isn't on the `load-path' by default.
bump to 0.9.0