update-my-copyright: bring over latest updates from OMPI script

Two major improvements from the OMPI version of this script:

1. Use better git methodology to find modified files.
2. Add some CLI options (e.g., you can use --check-only --quiet
   options in a git pre-commit hook)
netloc: make building netloc optional

Add --enable-netloc and --disable-netloc flags:

* --enable-netloc means: build netloc or fail configure if we can't
* --disable-netloc means: don't bother trying to build netloc

If neither is specified, if we can't build netloc, we'll skip it.