Check for strlcat and strlcpy

This partially reverts commit 8005ca4 by moving the strlcat()
and strlcpy() compatibility implementations back to their original
location.

In addition, these two functions were added to the AC_CHECK_FUNCS
macro. When these functions are available from the C library,
HAVE_STRLCAT and HAVE_STRLCPY will be defined and library version
used. Otherwise the compatibility version is built.

Reviewed-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8157
Closes #8202