Avoid crash caused by, and improve, 'findSpawnPos()' (#8728)

Avoid crash caused by, and improve, 'findSpawnPos()'  (#8728)

Avoid an unsuitable spawn position (which if outside mapgen limits can
cause a crash) if the main 0-3999 loop reaches its end. Fallback to a
spawn at 0,0,0.
Check the mapgen-returned 'spawn_level' value for being outside limits.
When 'air_count' reaches 2, move back down 1 to spawn in the lower
empty node.
If the spawn position is disallowed by 'objectpos_over_limit()', 'break'
from loop instead of 'continue' because positions above are probably
also over limit.
Reset 'air_count' to 0 if an obstruction is found, to make 'air_count'
consecutive empty nodes.
Allow spawn in 'airlike' drawtype nodes such as mod-added vacuum,
alien atmospheres, fog etc.
Add clarifying comments and improve codestyle.