Replace the CLI server's linear search for extensions with a hash table.

This doesn't make a statistically significant performance impact either way
today, but gives us more scope to grow the MIME type list.
Move the mime type map out of php_cli_server.c for easier generation.
Add a mime type map generation script and update the header.
Fix broken test.
Add NEWS. This doesn't need UPGRADING (or an RFC), IMO.