cve: Read entire CVE databases for the current year and the past year.

The "Modified" database that we were reading is much smaller, but it
only shows CVEs modified over the past week.

* guix/cve.scm (%now, %current-year, %past-year): New variables.
(yearly-feed-uri): New procedure.
(%cve-feed-uri, %ttl): Remove.
(%current-year-ttl, %past-year-ttl): New variables.
(call-with-cve-port): Add 'uri' and 'ttl' parameters and honor them.
Add 'setvbuf' call.
(current-vulnerabilities)[read-vulnerabilities]: New procedure.
Read from both %LAST-YEAR and %CURRENT-YEAR.