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

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

Because the argument to strvalid() is a size_t, use a size_t to iterate
over the array.

Submitted by: stefanf

Per permission from McAfee, remove clause 3 of the BSD license on
strvalid.c.

Use __FBSDID().

DBA update: used the wrong of several NAI DBAs.

Add a 'strvalid()' call to libkern. Given a character pointer, and
buffer length, determine if the pointer is to a valid string. Currently,
the only check is whether a '\0' appears in the buffer. This is useful
when pulling in a structure from userland that may contain one or more
strings, and validity testing must be performed on elements of the
structure. When copying normal string arguments, copyinstr() is
expected to be used.