url: move scheme detection to URL header/source

url: move scheme detection to URL header/source

Move enum url_scheme and url_get_scheme()
from connect.c to url.h and url.c
so that other code can identify
a URL's scheme without depending
on connect.c.

No behavior change. url_get_scheme() still dies
on an unrecognized scheme name, with the same
translated message as before.

scheme_name() stays in connect.c
because it has no other callers.

Signed-off-by: Matheus Afonso Martins Moreira <matheus@matheusmoreira.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>