doc: modernize libtool.texi.

doc: modernize libtool.texi.

* libtool.texi (@copying): Use this instead of @ifnottex; format for
Texinfo; eliminate obsolete paragraph about TeX; use @insertcopying
instead of duplicating text.
(@syncodeindex): Move all to the Texinfo header, where they belong.
(@dircategory): Use "Software development" instead of "GNU programming
tools", for consistency; and align individual entries to avoid
multiple lines; and move to after the copyright message.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>