viewed is used as a flag, i.e. bool. So treat is as such in most of the
places. vcs_vc is handled in the next patch.

Note: the last parameter of invert_screen was misnamed in the
declaration since 1.1.92.
Signed-off-by: NJiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200818085706.12163-3-jslaby@suse.czSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>