exec: command execution and process management

os::exec handles the execution of arbitrary commands. A programmer who needs to
spawn an external command will usually utilize [[cmd]] to obtain a [[command]]
instance (possibly using functions like [[setenv]] to customize it), then
[[start]] to execute it and obtain a [[process]] (or, [[exec]] to replace the
current process with it), then [[wait]] or [[peek]] to check in on its status.
