terminal adapter: make skipping of initial SIGSTOPs more reliable

first, _start being resolvable depends on libc-dbg being installed.
second, depending on the frame being in the dynloader makes it
a) work only for dynamic executables and b) fail on multi-target
systems (due to a hard-coded file name).
so instead just remember the entry point, as we are already there
anyway.

Reviewed-By: hjk