rman: Simplify initialization of internal globals

Use TAILQ_HEAD_INITIALIZER and MTX_SYSINIT to remove the 'once' code
from rman_init.

Reviewed by: des
Differential Revision: https://reviews.freebsd.org/D54142

(cherry picked from commit a35eae89cbddaac73d3b6c2b5c1b90e3a0c00997)