fix github issue #729 - edibility_prompts()

Issue #729 by argrath points out that one of the checks in
edibility_prompts() couldn't work.

For the next-meal effect after reading a blessed scroll of food
detection, the check whether a corpse was tainted but not dangerous
because the hero had sickness resistance could never be evaluated.
An earlier condition would cause the routine to return before
reaching that check.

Move it sooner, even though doing so violates the "order by most
to least dangerous" guideline.  It was either that or eliminate it
altogether.

edibility_prompts() had a lot of repetitive code that has now been
condensed.

Simplify acid blob corpse handling--for all of eating, not just for
edibility_prompts()--by treating that as "never rots" so that the
'rotted' variable always stays 0.  Now checks for that variable
being greater than some threshold don't need to include "and not an
acid blob corpse" as an exception.  A side-effect of this change is
that not only do they never become tainted, they'll no longer yield
the "you feel sick" outcome when they're old but not old enough to
exceed the tainted threshold.

Bug fix:  edibility testing stopped warning about green slimes.
That worked in 3.6.x, but 3.7 changed the 'cadaver' variable to
exclude them so the check for eating a glob of green slime could no
longer be reached.

Fixes #729

doc/fixes3-7-0.txt

@@ -1160,6 +1160,9 @@ items obtained via wishing while blind were erroneously being marked as 'seen
 	up close' so behaving differently from items picked up off the floor
 corpses of neuter monsters had random chance to be flagged as female so would
 	only stack with others that made the same random choice
+one-shot food testing ability conferred by blessed scroll of food detection
+	didn't warn about globs of green slime because the relevant code was
+	only executed for corpses, eggs, and tins, not for globs
 
 curses: 'msg_window' option wasn't functional for curses unless the binary
 	also included tty support