* proc: fix interaction of RequestManualStop and conditional breakpoints

A conditional breakpoint that is hit but has the condition evaluate to
false can block a RequestManualStop from working. If the conditional
breakpoint is set on an instruction that is executed very frequently by
multiple goroutines (or many conditional breakpoints are set) it could
prevent all calls to RequestManualStop from working.

This commit fixes the problem by changing proc.Continue to exit
unconditionally after a RequestManualStop is called.

* proc/gdbserial: fix ContinueOnce getting stuck on macOS

Fixes #902