Follow up to r225617. In order to maximize the re-usability of kernel code
in userland rename in-kernel getenv()/setenv() to kern_setenv()/kern_getenv().
This fixes a namespace collision with libc symbols.

Submitted by:   kmacy
Tested by:      make universe


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273174 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f