sys: Remove DEV_RANDOM device option

Remove 'device random' from kernel configurations that reference it (most).
Replace perhaps mistaken 'nodevice random' in two MIPS configs with 'options
RANDOM_LOADABLE' instead. Document removal in UPDATING; update NOTES and
random.4.

Reviewed by: delphij, markm (previous version)
Approved by: secteam(delphij)
Differential Revision: https://reviews.freebsd.org/D19918