Add a 'strvalid()' call to libkern. Given a character pointer, and

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.