xgettext: Support modifying the flags of a message after it has been remembered.

xgettext: Support modifying the flags of a message after it has been remembered.

* gettext-tools/src/xg-arglist-context.h (struct remembered_message_ty,
struct remembered_message_list_ty): New types.
(remembered_message_list_append): New declaration.
(struct formatstring_region_ty): Add a 'remembered' field.
* gettext-tools/src/xg-arglist-context.c (remembered_message_list_alloc,
remembered_message_list_append, remembered_message_list_ref,
remembered_message_list_unref): New functions.
(the_null_context_region): Update.
(inheriting_region): Inherit or allocate the 'remembered' fields.
(unref_region): Unref the 'remembered' fields.
* gettext-tools/src/xg-message.h (set_format_flag_from_context): New
declaration.
* gettext-tools/src/xg-message.c (validate_is_format): New function, extracted
from set_format_flags_from_context.
(set_format_flags_from_context): Change parameters. Invoke validate_is_format.
(set_format_flag_from_context): New function.
(remember_a_message, remember_a_message_plural): Update.