implement fork and vfork event support

DS2 does not report `fork-event` or `vfork-event` in the `qSupported` packet and does not implement these events. These events are necessary for lldb to support following child processes during a debug session. This missing support also results in several of the lldb tests failing when run against ds2 on Linux or Android (ex `TestGdbRemoteFork`).

Adding this support is likely non-trivial. This article gives a good overview of what was done to add this support to lldb on FreeBSD: https://www.moritz.systems/blog/lldb-support-for-fork-and-vfork/.