monitor and act on SYS_posix_spawn

In addition to treating most calls as fork (with the child pid stored in memory) we need to look at POSIX_SPAWN_SETEXEC which makes it behave like SYS_execve instead of SYS_fork, and also need to look at the signal state sharing flags and emulate them properly. We may also need to pay attention to the file sharing flags wrt our own files.

Xref #1643