Text clean-up of INSTALL.txt

Good idea to clean that up! Needs a bit more/different though... For instance, the paragraph starting with " So that the files directory ..." is not indented the same as the others around it, and once you do that, you'll find that the chmod line you fixed had the correct indentation after all.

Also, the bullet points in Section 6 - normally we align the bullets themselves with the paragraph before, and then indent the text for the bullets by two spaces. Text after bullet points should start with a capital letter also.

And there is inconsistency in bullets -- sometimes using * and sometimes using -. Should standardize on one or the other.

And as long as you are fixing this up, some of the menu nav has changed -- for instance, there is no longer a Configuration and Modules section.

Working on another issue, I just noticed that the INSTALL.txt file was still a mess, remembered there was an issue, and here I am.

So, here's a completely new patch:
- Fixes admin menu nav to match current Drupal 7
- Fixes wrapping so everything wraps at as close to 80 characters as possible (except one line that is longer -- the crontab doesn't fit on one line)
- Makes all bullets - instead of *, and follows our indenting/caps conventions with them
- Indents all commands/examples by 2 spaces.
- Fixes indentation of other stuff to be consistent

I also did one section of rewording - the bit about Clean URLs, which now points to the official Clean URLs section of the Handbook.

Also note this patch will conflict with the proposed fix in
#861192: Document that MariaDB works, too

#861192 resulted in a change of handbook page of http://drupal.org/requirements only and not in a patch to INSTALL.txt. So not really any conflict, but in the meantime there is a new more substantial new thread on changing INSTALL.TXT (http://drupal.org/node/952606), so I mark this one as duplicate.