If true, then display of user pictures or filenames will use the
method of providing arguments as "slash" arguments - this is much
better for caching, proxies, search engines etc. Unfortunately
it doesn't seem to work on some PHP installations.
If false (the default), then a more compatible method is used
(ie usual way of passing parameters to scripts)
rely on the administrator to tell us which is installed because PHP
is not reliable (eg GD 2.* functions are available in 4.0.6 and later
EVEN WHEN gd 2 is not present ... sigh).
So, new config variable $CFG->gdversion
