This adds a superset of the code from github pull request #328
to create a short-lived cloud of steam when fire hits a pool or
fountain. The original code required C99; this doesn't. It also
allowed vapor clouds on the Plane of Water where they don't work
sanely becaure regions don't understand air bubble movement and/or
vice versa. This inhibits the clouds there [the same ought to be
done for scrolls of stinking cloud]. It also left as-is the code
that reported when fountains got used up even though conceptually
the steam should interfere with being able to see that. This adds
a glyph-check hack to augment cansee() so that fountains that boil
away entirely are hidden at the time.
Regions that block line of slight should be calling block_point()
when created and unblock_point() when removed but a naive attempt
to introduce that didn't work as expected so I'm giving up on.
Fixes#328
274321d4ec