limits: Improve consistency

Historical precedent seems pretty consistent: size limits have singular
names, number limits have plural names. RLIMIT_VMM broke this, and I
made matters worse by referring to this limit as “vmms” in limits(1).
Consistently use “vms” everywhere user-visible, while leaving the
question of whether or not to rename RLIMIT_VMM itself for another day.

Fixes: 1092ec8b3375 ("kern: Introduce RLIMIT_VMM")
Fixes: 53af2026f213 ("limits: Unbreak after RLIMIT_VMM addition")
Reviewed by: bnovkov
Differential Revision: https://reviews.freebsd.org/D57265