Add -iglx & +iglx to Xserver.man

Covers the current state after commits 99f0365b1fbdfd9238b9f,
d0da0e9c3bb8fe0cd4879, & e3aa13b8d63ea2fba6eb4 were all applied.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: James Jones <jajones@nvidia.com>
Reviewed-by: Robert Morell <rmorell@nvidia.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
(cherry picked from commit b09d59342804db7dbb8056dca43dd39f54e290aa)

[alanc: Modified for server-1.16-branch to show +iglx as default instead of
        -iglx, to match code in os/utils.c in server-1.16-branch.]
Signed-off-by: Julien Cristau <jcristau@debian.org>