geom_label: Make glabel labels more trivial by separating the tasting routines out.

[delphij]

Summary

While there, also simplify the creation of label paths a little bit
by requiring the / suffix for label directory prefixes (ld_dir renamed
to ld_dirprefix to indicate the change) and stop defining macros for
these when they are only used once.

Test Plan

test glabel behavior after change

[delphij]

@cem -- Note that D24968 needs to be amended due to conflict (I have it locally with an amendment to D25168 to make geli detach -l work again with aliases), I've uploaded these to https://github.com/delphij/freebsd/commits/scratch/geom .

[cem]

In D25597#566273, @delphij wrote:

@cem -- Note that D24968 needs to be amended due to conflict (I have it locally with an amendment to D25168 to make geli detach -l work again with aliases), I've uploaded these to https://github.com/delphij/freebsd/commits/scratch/geom .

No worries, thanks for keeping it in mind.

This change looks good to me, sorry about the delay.

sys/geom/label/g_label.c

418–421 ↗

(On Diff #74224)

It might be nice to encode this in the g_labels[] table somehow instead.

[delphij]

ACK. I think this can be accomplished with a more intrusive change so let's do it in a later refactor instead.