nnml.el (nnml-request-accept-article): pass sender to

nnml.el (nnml-request-accept-article): pass sender to
nnmail-cache-insert

nnmh.el (nnmh-request-accept-article): pass sender to
nnmail-cache-insert

nnmbox.el (nnmbox-request-accept-article): pass sender to
nnmail-cache-insert

nnfolder.el (nnfolder-request-accept-article): pass sender to
nnmail-cache-insert

nnbabyl.el (nnbabyl-request-accept-article): pass sender to
nnmail-cache-insert

nnmail.el (nnmail-cache-insert): accept sender parameter and
pass it to the nnmail-spool-hook

gnus-registry.el (gnus-registry-track-extra): clarify doc
(gnus-registry-action): add sender lexical var and pass it to
gnus-registry-add-group
(gnus-registry-spool-action): take a sender parameter, pass to
gnus-registry-add-group
(gnus-registry-split-fancy-with-parent): trace by sender in
addition to subject
(gnus-registry-fetch-sender-fast): new function
(gnus-registry-add-group): accept sender parameter