ui_out_table: Replace boolean flag with enum

ui_out_table: Replace boolean flag with enum

This patch is just a little cleanup, it replaces the body_flag field of
ui_out_table with an enum.  It expresses more explicitly the
intent of the field (check that state == TABLE_STATE_HEADERS conveys
more what we want to do than checking for !body_flag).

New in v2:

  - Remove unnecessary ui_out_table_state::.

gdb/ChangeLog:

	* ui-out.c (enum ui_out_table_state): New enum.
	(struct ui_out_table) <body_flag>: Remove field.
	<state>: New field.
	(ui_out_table_begin): Replace usages of body_flag with state.
	(ui_out_table_body): Likewise.
	(ui_out_table_end): Likewise.
	(ui_out_table_header): Likewise.
	(ui_out_begin): Likewise.
	(verify_field): Likewise.
	(ui_out_new): Likewise.