Enhance realpath and dirname checking.

Enhance realpath and dirname checking.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_REALPATH_RETVAL): Do the checking
	in a function so that any core will show which test cored.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_REALPATH_RETVAL_OWNER): Ditto.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_REALPATH_ON_PROTECTED_MEMORY): Ditto.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_REALPATH_SANE_ON_NON_EXISTENT): Ditto.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_DIRNAME_SIDE_EFFECT): Ditto.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_DIRNAME_RETVAL_OWNER): Ditto.

	* m4/sxe-fs-funs.m4 (_SXE_CHECK_DIRNAME_RETVAL_OWNER): Avoid a
	core in MacOS X by pre-choosing sys as the result.

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>