Multiple -trusted/-untrusted/-CRLfile options in verify

It is sometimes useful (especially in automated tests) to supply
multiple trusted or untrusted certificates via separate files rather
than have to prepare a single file containing them all.

To that end, change verify(1) to accept these options zero or more
times.  Also automatically set -no-CAfile and -no-CApath when
-trusted is specified.

Improve verify(1) documentation, which could still use some work.

Reviewed-by: Richard Levitte <levitte@openssl.org>