Fix problems with serial comparison sed scripts, and factor out

Fix problems with serial comparison sed scripts, and factor out
common code from similar serial comparison functions:

* libtoolize.m4sh (func_keyword_update): Common code factored out
of func_ltmain_update and func_config_update.
(func_ltmain_update, func_config_update): Adjust, and fix sed
expressions to not use unportable ? modifier.
(func_install_update): New serial number based file updater keyed
of 'scriptversion' variable in install-sh.  Adjust all callers.