This avoids ~20s of XML parsing when running 'guix lint -c cve'.
* guix/cve.scm (vulnerability->sexp, sexp->vulnerability)
(fetch-vulnerabilities): New procedures.
(current-vulnerabilities): Use 'fetch-vulnerabilities'.
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.
Ludovic Courtès