SYNOPSIS
	string make_shared_string (string str)

DESCRIPTION
	Puts <str> in _the_ list of shared strings of the game.

	If the string is used by several variables/objects this
	saves memory. Keys of alists and mappings are always shared
	strings.

BUGS
	A better string handling in the driver should make this
	efun unnecessary.

HISTORY
	Introduced in LDMud 3.2.6; following a suggestion from TubMud.
