Proposal: rename min/max -> umin/umax

[Taylor R Campbell <campbell+netbsd-tech-kern%mumble.net@localhost>]

I propose we rename libkern's min/max to umin/umax.  The names min and
max invite general use like MIN and MAX, but these functions truncate
their arguments to unsigned first.  We also have imin/imax, lmin/lmax,
and ulmin/ulmax.

This came to my attention while working on a drmkms update, where
Linux's min/max are generic, and a quick grep found, for example:

https://nxr.netbsd.org/xref/src/sys/uvm/uvm_pglist.c#151

Objections?