vt: selection, split __set_selection_kernel

Handle these actions:
* poking console
* TIOCL_SELCLEAR
* TIOCL_SELMOUSEREPORT
* start/end precomputation
* clear_selection if the console changed
in a separate function, thus making __set_selection_kernel way shorter
and more readable. The function still needs dissection, but we are
approaching.

This includes introduction of vc_selection and renaming
__set_selection_kernel to vc_do_selection.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200415093608.10348-1-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>