printk: add CON_ALLDATA console flag

For consoles like netconsole and blockconsole the loglevel filtering
really doesn't make any sense.  If a line gets printed at all, please
send it down to that console, no questions asked.

For vga_con, it is a completely different matter, as the user sitting in
front of his console could get spammed by messages while trying to login
or similar.  So ignore_loglevel doesn't work as a one-size-fits-all
approach.  Add a per-console flag instead so that netconsole and
blockconsole can opt-in.

Signed-off-by: Joern Engel <joern@logfs.org>