GH650: Minor tidying around the ocsp app

The ocsp utility is something of a jack-of-all-trades; most anything
related to the OCSP can be done with it.  In particular, the manual
page calls out that it can be used as either a client or a server
of the protocol, but there are also a few things that it can do
which do not quite fit into either role, such as encoding an OCSP
request but not sending it, printing out a text form of an OCSP
response (or request) from a file akin to the asn1parse utility,
or performing a lookup into the server-side revocation database
without actually sending a request or response.  All three of these
are documented as examples in the manual page, but the documentation
prior to this commit is somewhat misleading, in that when printing
the text form of an OCSP response, the code also attempts to
verify the response, displaying an error message and returning
failure if the response does not verify.  (It is possible that
the response would be able to verify with the given example, since
the default trust roots are used for that verification, but OCSP
responses frequently have alternate certification authorities
that would require passing -CAfile or -CApath for verification.)

Tidy up the documentation by passing -noverify for the case of
converting from binary to textual representation, and also
change a few instances of -respin to -reqin as appropriate, note
that the -url option provides the same functionality as the -host
and -path options, clarify that the example that saves an OCSP
response to a file will also perform verification on that response,
and fix a couple grammar nits in the manual page.

Also remove an always-true conditional for rdb != NULL -- there
are no codepaths in which it could be initialized at the time of
this check.

Signed-off-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@openssl.org>