util/find-doc-nits: fixups

util/find-doc-nits: fixups

- Treat .pod.in files as well, and parse out the base name for those
  too.
- Correct the detection of the description part in the NAME section
  (the separating dash MUST be preceeded with a space)
- Allow slahes in names of the NAME section (convert them to dashes
  for file name comparison).  This allows manual pages for some of our
  header files, such as openssl/core.h.
- Properly detect repeated names in the NAME section.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9407)