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

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

Initialize fp_location for explicitly managed fail points, and push
the parentheses around the location for simple fail points into the
location string. This makes the print on fail point set more
consistent between the two versions.

Also fix up fail.h a little for style(9): only use one of sys/param.h
and sys/types.h, and use the existing __XSTRING() macro instead of
rolling our own. Also fix up a few tabs on changed and nearby lines.

Lastly, since KFAIL_POINT_{BEGIN,END} are not meant for use outside
this file, just eliminate the macros entirely.

MFC after: 1 week

Move the fail_point_entry definition from fail.h to kern_fail.c, which
allows putting the enumeration constants of fail point types with the
text string that matches them.

MFC after: 1 week

fail(9) support:

Add support for kernel fault injection using KFAIL_POINT_* macros and
fail_point_* infrastructure. Add example fail point in vfs_bio.c to
simulate VM buf pressure.

Approved by: dfr (mentor)