- The regexp for the "check if formula name is used in formula's
  description" cop matches every instance of the formula name if it
  exists, whether it's in a word or not.
- For example, the formula `mon` has the description "Monitor
  hosts/services/whatever and alert about problems". This makes
  `brew audit --strict` complain because it matches "Monitor",
  which isn't the formula name! The formula `pass` has the description
  "Password manager".  Again, the strict audit matches "Password",
  which isn't an issue.
- Instead, this change matches on a word boundary, so it will match
  `mon:`, or `mon `, but not "Monitor", or, for example, "harmony".
- I've changed the tests to account for this change.

Move some `brew doctor` checks that are pretty macOS specific so they
are only run on macOS.

This works around older versions of `which`, which return a
string on failure.

Ruby first gained flock in 1.8.7, which is a problem since we're using
this lock utility in `vendor-install` in order to install a newer Ruby.

Fortunately, Python 2.3(!) has flock support.