`vfork` is something of a relic, originally introduced as a (rather
unsafe) optimisation of `fork` in 3.0BSD, allowing the user to eliminate
the cost of cloning the parent process's address space (as early BSDs
would do) when `fork`ing with the intent of immediately `exec`ing.

However, its design has long been considered a mistake (being nigh
impossible to use safely in a multithreaded environment), its benefits
have largely vanished (since modern operating systems rely on virtual
memory to remap the parent's address space as copy-on-write instead of
copying), and nearly all platforms now consider it to be an alias of
`fork`.

Remove `process`'s support for `vfork`.

Fixes #261.