Compatibility with BSD version of `sed'

Rewrite calls to `sed' to remove '-r/--regexp-extended' parameter, which is not
supported by BSD `sed'.
BSD `sed' does implement regular expressions, but enables them with parameter
'-E' which in turn is not officially supported by GNU `sed' - in fact it is
implemented but not documented.

Also fix support for prompt strings (configure parameter
'--enable-root-menu-prompt') containing spaces.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>
Documentation: add MacOS X known-issues + typos

- Document MacOS X known-issues in the ad hoc file.
- Fix some typos in README file, and introduce a reference to
the known-issues file.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>