Make find-doc-nits check for newly added undocumented symbols

We create lists of undocumented functions and macros as they are now so
that find-doc-nits can check for newly introduced functions/macros that
are undocumented.

This works in a similar way to the -u and -d options to find-doc-nits.
These count undocumented symbols and print a detailed list of undocumented
symbols repsectively. This commit adds the -v and -e options to restrict
the count/detailed list to newly added undocumented symbols only.

There is also a new -s option that does the same as -e except that it
produces no output if there are no newly undocumented symbols.

We also amend "make doc-nits" to add the -s option which should cause
travis to fail if a PR adds undocumented symbols.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9094)