xgettext: Reduce code duplication.
* gettext-tools/src/xg-arglist-context.h (flag_context_list_table_add): Add
comment.
* gettext-tools/src/xg-arglist-context.c (set_flags_for_formatstring_type): New
function, extracted from flag_context_list_table_add.
(flag_context_list_table_add): Use it.
