Fix #10452: Don't let AyStar max_search_nodes unattended when initializing (#11544)

Add a constant with the default value of 10000 and have the pathfinding settings refer to it.

Add a preventative method to AyStar when it's initializing, to limit the number of max_search_nodes if left unattended.

Fix #7592: Do not cache road vehicle path within 8 tiles of destination with multiple entrances

Ported from jgrpp commit 79d5be7e265df3be8b73d484f0c7261b3c23229d

(svn r18481) -Codechange: unify the curve pathfinder penalty defaults; 0.01 tile won't make a dent, 3 tiles might be a bit too much
-Feature-ish: make maximum pathfinder penalties for finding depots customisable, also increase it slightly to 20 tiles worth of penalties.