RT 2820: Case-insensitive filenames on Darwin

Add darwin-*-cc as one of the systems for case-insensitive
filenames.  Fixes the manpage install so it doesn't create
looping symlinks.
Fix d4a4370050f7d72239b92a60ab9d4a2dd5e9fd84

Fully remove old error, per drH
Reviewed-by: rsalz
RT2609: Typo in EXAMPLE section of req.pod

The x509_extensions should be req_extensions in the
config example in req.pod

Reviewed-by: tjh@cryptsoft.com
Revert "RT 2820: Case-insensitive filenames on Darwin"

This reverts commit 691edc997a35682eb7fa29445036182d2c9eb1de.
PR 2580: dgst missing current SHA algorithms

Update the dgst.pod page to include SHA224...512 algorithms.
Update apps/progs.pl to add them to the digest command table.

Reviewed-by: Tim Hudson <tjh@cryptosoft.com>