connect: rename enum protocol to url_scheme

connect: rename enum protocol to url_scheme

RFC 1738 names the part of a URL before the colon a "scheme".
connect.c calls it "protocol", which is more generic
and collides with the unrelated enum protocol_version.

Rename:

    enum protocol -> enum url_scheme
    PROTO_*       -> URL_SCHEME_*
    prot_name     -> url_scheme_name
    get_protocol  -> url_get_scheme

The local variables in parse_connect_url and git_connect
are renamed accordingly, from protocol to scheme.

No behavior change. The user-visible diagnostics
and translated error messages are preserved:

    "Diag: protocol=..."
    "protocol '%s' is not supported"
    "unknown protocol"

This rename also prepares for moving the scheme-detection functions
to a shared header so that a future plumbing command can parse URLs
using the same logic as the connect path.

Suggested-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Matheus Afonso Martins Moreira <matheus@matheusmoreira.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>