rng-tools: uci-fy, default to doing nothing

rng-tools: uci-fy, default to doing nothing

Enabling the default configuration will pipe /dev/urandom back into
/dev/random ala the current package behavior.  Because this amounts to
disabling the in-kernel entropy estimation, default disabled.

While here, uci-paramaterize the high watermark.
While here, add a pre-command hook for real RNGs that need 'stty raw'
or other such hooks.  (e.g. the TrueRNG devices)

As offered in github openwrt/packages#3422, take over maintainership.

See github openwrt/packages#3142.

Signed-off-by: Nathaniel Wesley Filardo <nwfilardo@gmail.com>