string: provide strscpy()

The strscpy() API is intended to be used instead of strlcpy(),
and instead of most uses of strncpy().

- Unlike strlcpy(), it doesn't read from memory beyond (src + size).

- Unlike strlcpy() or strncpy(), the API provides an easy way to check
  for destination buffer overflow: an -E2BIG error return value.

- The provided implementation is robust in the face of the source
  buffer being asynchronously changed during the copy, unlike the
  current implementation of strlcpy().

- Unlike strncpy(), the destination buffer will be NUL-terminated
  if the string in the source buffer is too long.

- Also unlike strncpy(), the destination buffer will not be updated
  beyond the NUL termination, avoiding strncpy's behavior of zeroing
  the entire tail end of the destination buffer.  (A memset() after
  the strscpy() can be used if this behavior is desired.)

- The implementation should be reasonably performant on all
  platforms since it uses the asm/word-at-a-time.h API rather than
  simple byte copy.  Kernel-to-kernel string copy is not considered
  to be performance critical in any case.

Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>