utf8.c: Add UTF-8 validation and utility functions

There are various places in Asterisk - specifically in regards to
database integration - where having some kind of UTF-8 validation would
be beneficial. This patch adds:

* Functions to validate that a given string contains only valid UTF-8
  sequences.

* A function to copy a string (similar to ast_copy_string) stopping when
  an invalid UTF-8 sequence is encountered.

* A UTF-8 validator that allows for progressive validation.

All of this is based on the excellent UTF-8 decoder by Björn Höhrmann.
More information is available here:

    https://bjoern.hoehrmann.de/utf-8/decoder/dfa/

The API was written in such a way that should allow us to replace the
implementation later should we determine that we need something more
comprehensive.

Change-Id: I3555d787a79e7c780a7800cd26e0b5056368abf9