Move rump kernel man pages from various sources to sys/rump

namely:
  * src/lib is used only when building for POSIX'y platforms, but
    the man pages have their use for all platforms
  * rumpuser.3 is a function of the rump kernel, not one of the of
    the POSIX'y implementation hosted in src/lib/librumpuser

no functional change
Do not uselessly include <sys/malloc.h>.
Add Realtek RTL8251.
regen
match Realtek RTL8251.