Move CVE information into advisories directory

One of the requirements to becoming a CVE Numbering Authority (CNA) is
to publish advisories.  Do this by maintaining a file for each CVE fixed
in the advisories directory in the source tree.  Links to the advisories
can then be shared as:

https://sourceware.org/git/?p=glibc.git;a=blob_plain;f=advisories/GLIBC-SA-YYYY-NNNN



The file format at the moment is rudimentary and derives from the git
commit format, i.e. a subject line and a potentially multi-paragraph
description and then tags to describe some meta information.  This is a
loose format at the moment and could change as we evolve this.

Also add a script process-fixed-cves.sh that processes these advisories
and generates a list to add to NEWS at release time.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>