* lisp/net/pop3.el: Use lexical-binding and cl-lib.
(pop3-write-to-file): Remove unused var 'start'.
(pop3-make-date): Remove unused var 'sign'.
* lisp/net/rfc2104.el: Use lexical-binding and cl-lib.
* lisp/net/shr-color.el: Use lexical-binding and cl-lib.
* lisp/net/sieve-manage.el: Use lexical-binding and cl-lib.
Most of this change is to boilerplate commentary such as license URLs.
This change was prompted by ftp://ftp.gnu.org's going-away party,
planned for November. Change these FTP URLs to https://ftp.gnu.org
instead. Make similar changes for URLs to other organizations moving
away from FTP. Also, change HTTP to HTTPS for URLs to gnu.org and
fsf.org when this works, as this will further help defend against
man-in-the-middle attacks (for this part I omitted the MS-DOS and
MS-Windows sources and the test tarballs to keep the workload down).
HTTPS is not fully working to lists.gnu.org so I left those URLs alone
for now.
