Merge branch 'setup-fixes' into 'next'

Fixes and changes related to the act of setting up a level (in other words, setup fixes)

Changes made in this branch so far:
* a REJECT lump of zero length (NOT to be confused with a REJECT lump of non-zero length with all-zeros, just to clarify) should no longer cause problems with netgames and otherwise. The game now checks if the lump exists, has the right name, and length is not zero - if it fails any of these, the REJECT lump is not loaded and P_CheckSight won't be allowed to use it. This means ZDBSP (no reject) should be safe(r) to use now!
* there's now a simple devmode-only message (requires "Setup" mode) if a sector is found to have no lines at all during setup. It's a far cry from the plot to I_Error the game I once had for that unsusal scenario, but such a level still works anyway so whatever.

See merge request !87