The reboot() system call accepts a mode (RB_AUTOBOOT, RB_HALT, RB_POWEROFF,
or RB_REROOT) as well as zero or more optional flags in 'howto'.
However, RB_AUTOBOOT was only displayed if 'howto' was exactly 0.
Combinations like 'RB_AUTOBOOT | RB_DUMP' were decoded as 'RB_DUMP'.
Instead, imply that RB_AUTOBOOT was specified if none of the other "mode"
flags were specified.