Rename OF_xref_phandle() to OF_node_from_xref() and add a new function
that provides the inverse translation, OF_xref_from_node().

Discussed with:	nwhitehorn
According to the ISO C standard, lgamma(-integer) returns
inf and raises the divided-by-zero exception.  Compilers
constant fold one/zero to inf but do not raise the exception.
Introduce a volatile vzero to prevent the constant folding.

Move the declaration of zero into the main declaration block.

While here, fix a nearby disordering of 'lx,ix'

Discussed with:	bde
sdhci.h has grown a dependency on sysctl.h, include the latter where needed.