This feature makes it possible to toggle the gdump feature on/off during
program execution, whereas the the opt.prof_dump mallctl value can only
be set during program startup.
This resolves#72.
Fix a regression in prof_dump_ctx() due to an uninitized variable. This
was caused by revision 4f37ef693e3d5903ce07dc0b61c0da320b35e3d9, so no
releases are affected.