Enable Host::LaunchProcess on Linux
- move LaunchProcessPosixSpawn() and Host::LaunchProcess() from freebsd host plugin to common (linux/freebsd section)
- modify MonitorChildProcessThreadFunction to use pid_t from sys/types.h to avoid Linux/FreeBSD/Mac warnings when calling waitpid()



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@189404 91177308-0d34-0410-b5e6-96231b3b80d8