MINOR: http: add a new function http_validate_scheme() to validate a scheme

MINOR: http: add a new function http_validate_scheme() to validate a scheme

While http_parse_scheme() extracts a scheme from a URI by extracting
exactly the valid characters and stopping on delimiters, this new
function performs the same on a fixed-size string.

(cherry picked from commit d3d8d03d98c6624c8b15b81f7b80dd22fc3f9fd2)
[wt: context adj]
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit a12054991079cfb6c0f8b7484a8b8209be785cd2)
[wt: context adj]
Signed-off-by: Willy Tarreau <w@1wt.eu>