Don't ever make time slow down unless there are hostiles present.

After all, what's the use of it? So now, the slider being lower than
max doesn't mean that you have to wait for all your shields to come
back before time will speed up when there aren't even any enemy ships
around.
Fixed autonav time resetting to work with *all* hostiles.

The previous method only worked for *enemies*, which meant that
other hostiles (normally generated by mission scripts) were being
ignored. The new method takes all hostiles into account.
Require damage for slowdown when slider is below "enemy presence".

I also added a 2-second delay after time is reset, before speeding
up again, to give you time to react when you take damage. A minor
side effect of this is those times when enemy presence is detected
for one frame, which I think is incorrect (maybe due to the way ships
spawn?), are going to be much more noticeable.

Conflicts:

	src/player_autonav.c
Added a fix for those times when ships are spotted for one frame.

I think that is a result in a defect in spawning of ships. Anyway,
I worked around it by requiring enemy presence for two frames in a
row to trigger time reset. It works as it should. This makes the
system much smoother.

Also made it so the delay for speeding up time again is only used
for damaged-based time resets.
Some small improvements to autonav speed resetting.
Fixed a typo.
Merge branch 'speedfix' of https://github.com/onpon4/naev into tmp
Added a hint about autonav speed reset.

Conflicts:

	dat/events/neutral/npc.lua
Removed unnecessary stuff from mission.xml.

There were a few empty <cond> tags and self-closing <flags/> tags.
These are useless, so I removed them.
Merge branch 'bugfix' of https://github.com/onpon4/naev into tmp