libtoolize: fix some long-standing sed substitution bugs

libtoolize: fix some long-standing sed substitution bugs

* libtoolize.m4sh (func_fixup_Makefile): `\\\\\$' in a " quoted
script that is evaluated twice before execution becomes a `\$',
which sed interprets as a literal $.  The script will then delete
any lines with a first non-whitespace character of `$'. Instead
use `\\\\$' in single quotes, which sed eventually receives as
`\\$' and correctly interprets as part of an instruction to
delete otherwise empty lines ending with a `\'.
Additionally the rest of the same sed script is no longer
applied to comment lines to avoid munging the copyright header
among others.
* NEWS: Updated.