set 'bouldersym' via 'O'

set 'bouldersym' via 'O'

Reinstate the deprecated 'bouldersym' option for the 'O' command
instead of just showing the set-at-start-of-play value.  I leave
boulder with the default value except when attempting to solve
Sokoban, and it's a pain to have to save, edit config file or
reset NEHTACKOPTIONS, then restore in order to change the boulder
symbol, then do that all over again once leaving Sokoban.

If using the default symbols, a custom symbol set which just sets
boulder will work, but with {DEC|IBM}graphics that's no help
because the unspecified symbols revert to their default values.