Undocumented get_flag options

I've just been browsing the docs & implementations of get_flag and set_flag.
There seem to be a number of inconsistencies.

The flag `default_language' is supported by both get_flag and set_flag, but
is not documented.

The flags `max_global_trail' and `max_local_control' are documented and
returned by get_flag, but set_flag does not recognise them (other read-only
flags print a message saying they're read only rather than giving a generic
out-of-range error).

A number of flags are valid for set_flag, but are not supported by get_flag
and are not documented.  Some are obsolete, and are simply ignored or print
an error.  They are:

	debugger_warnings
	default_language
	gc_incr
	global_size (prints error)
	load_release_delay
	max_vars_per_clause (ignored)
	parser_size (ignored)
	publishing_parameter
	statistics (obsolete)
	strip_variables (can only turn on!?!?)
	user_options (ignored)

Strangely, `strip_variables' can only be turned on.  I intend to fix this
one (since I want to be able to turn it off for the IC tests), and while I'm
at it I'll fix `max_global_trail' and `max_local_control'.  The rest I'll
claim I don't know enough about to tamper with the week before a release.
:)

Cheers,
Warwick