Cleanup in_ifscrub(), which is just an entry to in_scrubprefix().
in_ifadown() can be void.
Make getenv_*() functions and respectively TUNABLE_*_FETCH() macros not
allocate memory and so not require sleepable environment.  getenv() has
already used on-stack temporary storage, so just use it more rationally.
getenv_string() receives buffer as argument, so don't need another one.
Remove the dependency on procfs.

Reviewed by:	kib
MFC after:	1 week
X-MFC with:	r257430