libtracefs: Add tracefs_event_is_enabled() API

Add a function that checks if an event (or events) are enabled or not.
Returns the enums:

  TRACEFS_ERROR		= -1
  TRACEFS_ALL_DISABLED	= 0
  TRACEFS_ALL_ENABLED	= 1
  TRACEFS_SOME_ENABLED	= 2

Link: https://lore.kernel.org/linux-trace-devel/20221114204522.2433500-2-rostedt@goodmis.org

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>