RT2880: HFS is case-insensitive filenames

Add Darwin to list of case-insensitive filenames when
installing manapges.  When doing this, I noticed that
we weren't setting "filecase" for the HTML doc install.

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>
Fixed double inclusion of string.h

PR2693

Reviewed-by: Tim Hudson <tjh@openssl.org>