Slight cleanup of the collection of READMEs, INSTALLs and NOTES

README is a fairly independent document, and so is INSTALL.  NOTES are
merely addendums to INSTALL.  Therefore , INSTALL.DJGPP and
README.PERL get renamed to NOTES.DJGPP and NOTES.PERL.

Reviewed-by: Rich Salz <rsalz@openssl.org>