Strip trailing / characters from paths in "not present" index entries, not
just "directory" entries.

Prior to this commit, if / was added as part of a security update (how? In
the most recent case, because lib32 was accidentally omitted and was then
re-added, and every installer distribution set gets its own paths) then
the code which was supposed to filter out updates to deleted parts of the
base system (if someone decides to delete / then we shouldn't re-create it
for them) would instead get confused and decided that while / should exist,
// should not exist and needs to be removed.

This fixes the bug which caused freebsd-update to want to delete / (which is
harmless, since `rm /` fails, but scary nonetheless).  A workaround is being
applied to the update bits in order to avoid triggering the bug on unpatched
systems.

PR:		196055, 196091, 196147
Add a new loader(8) variable, twiddle_divisor, allowing control over the
output frequency of the "twiddle" IO progress indicator.  The default
value is 1.  For larger values N, the next stage of the animation is only
output on every Nth call to the output routine.  A sufficiently large N
effectively disables the animation completely.