fortify: Add protection for strlcat()

The definition of strcat() was defined in terms of unfortified strlcat(),
but that meant there was no bounds checking done on the internal strlen()
calls, and the (bounded) copy would be performed before reporting a
failure. Additionally, pathological cases (i.e. unterminated destination
buffer) did not make calls to fortify_panic(), which will make future unit
testing more difficult. Instead, explicitly define a fortified strlcat()
wrapper for strcat() to use.

Signed-off-by: Kees Cook <keescook@chromium.org>