The OCSP responder URI included within an X.509 certificate may or may
not include a trailing slash.  We currently rely on the fact that
format_uri() incorrectly inserts an initial slash, which we include
unconditionally within the OCSP request URI.

Switch to using uri_encode() directly, and insert a slash only if the
X.509 certificate's OCSP responder URI does not already include a
trailing slash.

Signed-off-by: Michael Brown <mcb30@ipxe.org>