Add support executing gdb command synchronuously.

It's a bit ridiculous to workaround async stuff this way, but I'm a bit
too shy to kill it right away. But any more or less advanced asyn usage
leads to callback spaghetti, so leave current async usage as is, and target
to use sane sync calls otherwise.
Reload disasm when switching function frames.

This relies on sync gdb command execution.
Load next disasm chunk when stepping thru code too.