Use XORG_TOO_OLD macros instead of NXAGENT_SERVER for missing Xserver features

[sunweaver]

Throughout the Xserver code, we use defined(NXAGENT_SERVER) to check if building nxagent.

When backporting extensions, we need this check and it denotes that actually the underlying Xserver code (mainly in Xserver/dix/ and Xserver/mi/) is too old for recent extension ABIs.

We should use another macro name for indicating a too old Xserver version, e.g. XORG_TOO_OLD.

See #125, #137 for earlier discussions.