Rename 'cylinder.c' as 'equipment.c'

Rename 'cylinder.c' as 'equipment.c'

Make it about general equipment management, and start hooking up
functions to show new equipment information when changing dives (and to
flush changes to equipment information for the previously active dive).

Nothing is hooked up yet, and it's now showing just one (really big)
cylinder choice, so this is all broken.  But it should make it possible
to at least get somewhere some day.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>