*.h: move some *_INIT to designated initializers

Move various *_INIT macros to use designated initializers. This helps
readability. I've only picked those leftover macros that were not
touched by another in-flight series of mine which changed others, but
also how initialization was done.

In the case of SUBMODULE_ALTERNATE_SETUP_INIT I've left an explicit
initialization of "error_mode", even though
SUBMODULE_ALTERNATE_ERROR_IGNORE itself is defined as "0". Let's not
peek under the hood and assume that enum fields we know the value of
will stay at "0".

The change to "TESTSUITE_INIT" in "t/helper/test-run-command.c" was
part of an earlier on-list version[1] of c90be786da (test-tool
run-command: fix flip-flop init pattern, 2021-09-11).

1. https://lore.kernel.org/git/patch-1.1-0aa4523ab6e-20210909T130849Z-avarab@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list.h

@@ -46,7 +46,10 @@ struct list_head {
 #define INIT_LIST_HEAD(ptr) \
 	(ptr)->next = (ptr)->prev = (ptr)
 
-#define LIST_HEAD_INIT(name) { &(name), &(name) }
+#define LIST_HEAD_INIT(name) { \
+	.next = &(name), \
+	.prev = &(name), \
+}
 
 /* Add new element at the head of the list. */
 static inline void list_add(struct list_head *newp, struct list_head *head)