Add haddock comments on RTLD_NEXT and RTLD_DEFAULT

Related ticket: #8902.
Remove unnecessary checks for RTLD_NOW and RTLD_GLOBAL

These checks were introduced for OpenBSD on July 16 2002 in GHC commit
03e9edb3094fd3bb38ed886b96ee9f61f39e9b53.

According to http://www.openbsd.org/cgi-bin/cvsweb/src/include/dlfcn.h
RTLD_NOW, RTLD_GLOBAL and RTLD_LOCAL have been available on OpenBSD
since Revision 1.8 (September 2 2003).

This is merely code cleanup.
Deprecate function `haveRtldLocal`

The function haveRtldLocal was introduced for compatibility with Cygwin
on Mar 28 2002 in GHC commit 4740cf56c774b92e02d31b4666158d70c2e85a8f.

According to https://cygwin.com/viewvc/src/winsup/cygwin/include/dlfcn.h
RTLD_LOCAL has been available on Cygwin since Revision 1.4 (August 9 2010).
Merge pull request #2 from thomie/T8902

Add haddock comments on RTLD_NEXT and RTLD_DEFAULT