More FORTIFY_SOURCE functions under clang

* bzero
* umask
* strlcat

Change-Id: I65065208e0b8b37e10f6a266d5305de8fa9e59fc
FORTIFY_SOURCE: introduce __strncpy_chk2

This change detects programs reading beyond the end of "src" when
calling strncpy.

Change-Id: Ie1b42de923385d62552b22c27b2d4713ab77ee03
cdefs.h: introduce __bos0

Introduce __bos0 as a #define for __builtin_object_size((s), 0).
This macro is intended to be used for places where the standard
__bos macro isn't appropriate.

memcpy, memmove, and memset deliberately use __bos0. This is done
for two reasons:

1) I haven't yet tested to see if __bos is safe to use.
2) glibc uses __bos0 for these methods.

Change-Id: Ifbe02efdb10a72fe3529dbcc47ff647bde6feeca
Fix unnecessary call to __strncpy_chk2

If "n" is smaller than the size of "src", then we'll
never read off the end of src. It makes no sense to call
__strncpy_chk2 in those circumstances.

For example, consider the following code:

int main() {
  char src[10];
  char dst[5];
  memcpy(src, "0123456789", sizeof(src));
  strncpy(dst, src, sizeof(dst));
  dst[4] = '\0';
  printf("%s\n", dst);
  return 0;
}

In this code, it's clear that the strncpy will never read off
the end of src.

Change-Id: I9cf58857a0c5216b4576d21d3c1625e2913ccc03