c-common.c (c_common_truthvalue_conversion): Generalise warning for addresses converted to booleans; not just function addresses.

	* c-common.c (c_common_truthvalue_conversion): Generalise warning
	for addresses converted to booleans; not just function addresses.
	* c-typeck.c (build_binary_op): Warn for address comparisons which
	can never be NULL (eg. func == NULL or &var == NULL).
	* common.opt (Walways-true): New option.
	* c-opts.c (c_common_handle_option): Set it with -Wall.
	* doc/invoke.texi: Document it.
testsuite/
	* gcc.dg/warn-addr-cmp.c: New test.

From-SVN: r108489