Several tweaks to the install.php script

- Fix description of the "admin" option, which says there's a default,
  "WikiSysop", when this isn't the case.
- Use "MediaWiki" instead of "Don't care" as the default value
  for the "name" argument
- make the "name" argument optional, so that its default value
  can actually work as such
- test for getOption's default parameter value (null)
  rather than providing one (false) and testing for it
- fix indication of default value for --dbpath
  and document where it comes from
- format the default for --confpath
  the same way as other defaults are displayed
- add description
- use trim() for --passfile, as is done for --dbpassfile
- use single quotes where possible
- mention in the documentation where the default values come from
- various minor text adjustments
- bonus: typo fix in a comment in Maintenance.php

Change-Id: Icbc0b02cfbf09b3f97ed86f4a68c3b35a9c839c8