Proposal for new TS plugin API function: TSUrlPortGet

[Walt Karas]

This gets the port string in the URL. If the port is not in the URL, it
returns nullptr (and sets length to zeor). Note this is subtly different
from TSUrlPortGet which always returns a valid port, the canonical port for
the scheme if the port is not specified. Currently there is no way to
detect this from a plugin.

char const* TSUrlPortGet(TSMBuffer, TSMLoc, int* length);