Avoid atexit(3) when possible, disable prof_final by default.

atexit(3) can deadlock internally during its own initialization if
jemalloc calls atexit() during jemalloc initialization.  Mitigate the
impact by restructuring prof initialization to avoid calling atexit()
unless the registered function will actually dump a final heap profile.

Additionally, disable prof_final by default so that this land mine is
opt-in rather than opt-out.

This resolves #144.