Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

Update share/examples/* to properly install /usr/share/examples.

As result of this, a new examples package is now created.

Note, this is only effective with 'SHARED=copies' (the default),
as the 'SHARED=symlinks' mechanism will create a symlink to the
source tree version of the file(s).

Sponsored by: The FreeBSD Foundation

- Use libusb20_strerror() function instead of custom usb_error() one.
- Rename "aux.[ch]" to "util.[ch]" which is a more common name for
utility functions and allows checkout on some non-FreeBSD systems
where the "aux.*" namespace is reserved.
- Fix some compile warnings while at it.

PR: usb/183728
MFC after: 2 weeks

Add some examples about how to use FreeBSD's libusb20 in your own
code.