config_["hide-vacant"].asBool() was called from the river status listeners.
When the option is given as a string ("true") jsoncpp's asBool() throws
"Value is not convertible to bool", and that exception unwinding through
libwayland's C dispatch aborts the process. Parse it once in the constructor
into a bool member (accepting the string form) and read the cached value.
Fixes#4078
Alex
699589b66a