Improve shr/eww handling of mailto URLs

Improve shr/eww handling of mailto URLs

* lisp/net/eww.el (eww): Use function-put in place of put, as
recommended in "(elisp) Symbol Plists".
(eww-follow-link):
* lisp/net/shr.el (shr-browse-url): Rather than call browse-url-mail
directly, call browse-url which respects the user options
browse-url-handlers and browse-url-mailto-function.  (Bug#41133)
(shr--current-link-region): Return nil if there is no link at point.
(shr--blink-link): Adapt accordingly.
(shr-fill-line, shr-indent, shr-table-body): Refactor to avoid some
unnecessary allocations.
* etc/NEWS: Announce that eww-follow-link and shr-browse-url support
custom URL handlers.