Typos and minor edits of messages (part 1)

 - README.TXT: minor edit
 - src/doinst.sh: minor change to postinstall message
 - src/repositories.txt: minor edits
 - src/slack-desc: use same text as README.TXT; tweak handy-ruler
 - src/slackpkgplus.sh: minor edits in comments and echo messages
Typos and minor edits (part 2)

 - src/README: Typos, reword a few paragraphs
Typos and minor edits (part 3)

 - setupmultilib.sh: typos in echo messages
Typos and minor edits (part 4)

 - src/slackpkgplus.x86.sample and src/slackpkgplus.x86_64.sample:
   minor edits in comments
src/README: reformatted.
Thanks to idlemoor for typos correction.