url: return URL_SCHEME_UNKNOWN instead of dying

url: return URL_SCHEME_UNKNOWN instead of dying

Enumerate a URL_SCHEME_UNKNOWN result with value 0.
Have url_get_scheme() return it for unrecognized
schemes instead of calling die() itself.
Move the die() call to parse_connect_url()
where url_get_scheme() is used.

This lets url_get_scheme() be used from contexts
that need to identify a URL's scheme without aborting
the program. For example, a future plumbing command
that validates URLs.

No external behavior change. parse_connect_url() still dies
with the same translated message for unrecognized schemes.

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