nudgeoutofsolid: implement fallback for when cliphulls are in use

nudgeoutofsolid: implement fallback for when cliphulls are in use

Fixes sv_gameplayfix_droptofloorstartsolid_nudgetocorrect and
DP_QC_NUDGEOUTOFSOLID doing nothing on Q1BSP.

Fixes the Q1BSP condition in PHYS_NudgeOutOfSolid() to handle the exact
issue (so mod_q1bsp_polygoncollisions is properly supported).

Migrates unsticking code from server-specific to common.

Updates documentation.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>