Remove opt_random.h header pollution from sys/random.h by moving
RANDOM_LOADABLE and RANDOM_YARROW's definitions from opt_random.h to
opt_global.h

This unbreaks make depend in sys/modules with multiple drivers (tmpfs, etc)
after r286839

X-MFC with: r286839
Reviewed by: imp
Submitted by: lwhsu
Differential Revision: D3486