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.

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

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