New & improved man page attached, now with standard license added.

New & improved man page attached, now with standard license added.
Plus, a patch for scan-build.

* mdoc corrections
* slightly more compact output
* same license as scan-build
* DESCRIPTION describes
* Default checkers corrected & explained
* Authors credited

The patch adds support for --help-checkers.  It just lists the default
checkers by recursively invoking "scan-build -h" and looking for the
magic '+' signs.

Patch by James Lowden!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157411 91177308-0d34-0410-b5e6-96231b3b80d8