debuginfod: Document and sanity check debuginfod_add_http_header format.

debuginfod: Document and sanity check debuginfod_add_http_header format.

Document and sanity check the format of the header string form that can
be passed to debuginfod_add_http_header. It should contain precisely
one colon, which cannot be the first or last character. And the function
should only be used to add optional headers, not replace any existing
standard ones. Anything else isn't supported.

Signed-off-by: Mark Wielaard <mark@klomp.org>