Improve cleanup after detaching heaptrack

Improve cleanup after detaching heaptrack

We now stop heaptrack explicitly via GDB before breaking the pipe.
This improves the stability of the unload process of heaptrack
after runtime-injection. It is still somewhat shaky though. I still
see crashes on shutdown of the debuggee now, long after heaptrack
was stopped. Similarly, I see tons of issues when I try to unload
libheaptrack_inject via __libc_dlclose, so I'm keeping the library
loaded for now.

I've added an explicit warning now to the `-p` option, saying that
runtime-attaching is unstable and can crash the debuggee.

BUG: 377143