Add os/posix-exec (along os/posix-fork)

Useful for old-style unix daemons, start up scripts, and so on.
Easy to add on top of os/execute.

May want to consider allowing the same IO redirection as os/execute
and os/spawn.

May also want to put both fork and exec behind a config switch since I
suppose some systems may not support them, although I don't know of any
concrete examples.

examples/posix-exec.janet

@@ -0,0 +1,5 @@
+# Switch to python
+
+(print "running in Janet")
+(os/posix-exec ["python"] :p)
+(print "will not print")