Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Change ofwdump(8) to use sysexits(3) exit codes.
- Make the code use the new OFIOCMAXVALUE instead of defining the maximum
length of property values locally.
- Move the application specific parts from ofw_util.c to ofwdump.c in
order to make ofw_util.c more library-like. While ofw_dump_properties()
could be made non-specific to ofwdump(8) it's currently optimized for
use in ofwdump(8) and making it a library-like function would just
complicate the code unnecessarily.
- Minor clean-up in ofw_util.c, e.g. make its use of getopt(3) the way
it's described in style(9), make its usage() static, etc.
- Add a comment in ofw_util.c about why it doesn't call usage() when
neither the "-a" option nor a node-name where given.
- Add ofw_optnode() and ofw_setprop(), helper functions for the
OFIOCGETOPTNODE and OFIOCSET ioctls respectively, to ofw_util.[c,h].
- Be consistent with the use of 'const' in ofw_util.[c,h] and add 'const'
to the function arguments that are acutally const but weren't declared
as such.
- Mark WARNS=6 clean.

Approved by: tmm

Use void * instead of char * to avoid violating C99 strict aliasing rules.

Autosize the property buffer instead of limiting it to 8192, and
exiting if a property happens to be larger.

Increase the size of the property buffer to 8192. 1024 wasn't enough to
retrieve certain nodes.

Add a utility to examine the OpenFirmware device tree (on sparc64). This
allows access to detailed machine configuration information, and should
be especially useful to gather information for driver-related bug
reports.