strlcpy.[ch]: Add strlcpy_()

strlcpy.[ch]: Add strlcpy_()

This function is like strlcpy(3), but returns -1 on truncation, which
makes it much easier to test.  strlcpy(3) is useful in two cases:

-  We don't care if the output is truncated.  strlcpy(3) is fine for
   those, and the return value can be ignored.

-  Truncation is bad.  In that case, we just want to signal truncation,
   and the length of the original string is quite useless.  Return the
   length iff no truncation so that we can use it if necessary.

This simplifies the definition of the STRLCPY() macro.

Signed-off-by: Alejandro Colomar <alx@kernel.org>