Continued profiler loading cleanup (PR#2816 follow up)

Continued profiler loading cleanup (PR#2816 follow up)

- Back out MONO_PROFILER_LIB_DIR change
- Profiler loading traces should use log mask “dll” not “asm”
- mono_dl_open_runtime_lib should preempt standard paths when loading
  profiler
- mono_dl_open_runtime_lib should check in profiler library directory
- mono_dl_open_runtime_lib contained a memory leak
- With above changes, ptestrunner.pl can remove some environment
  massaging