Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nnimap.el (nnimap-transform-headers): Protect against (NIL ...) bodystructures (bug#9314).
starttls.el (starttls-any-program-available): Define as obsolete function.
gnus-util.el (gnus-y-or-n-p): Reinstate the message-clearing y-or-n-p versions which Gnus use when appropriate.
gnus-group.el (gnus-group-clear-data): Add a y-or-n query, since it's a pretty destructive command.
nnmail.el (nnmail-extra-headers): Clarify slightly (bug#9302).
message.el (message-fix-before-sending): Make a different warning about NUL characters (bug#9270).
gnus-sum.el (gnus-auto-select-subject): Allow specifying a function from custom (bug#9260).