Issue reported by AmyBSOD: several actions change the object type of
a potion rather than force creation of a replacement one, and if/when
the type was changed to oil, the age wasn't converted from absolute
to relative. Relative age is the amount available and/or the number
of turns it will burn if applied. The later in a game a potion got
converted into oil, the longer it would burn. Not mentioned: reverse
situation was also the case, although that didn't have any noticeable
effect since incorrect absolute age of former oil doesn't matter.
Not thoroughly tested. I got a potion of oil from a horn of plenty
and it burned for 400 turns, but it might have been created directly
rather than be a rejected magic potion that was converted into oil.
Closes#1191
c6897bf331