Add new Process subsystem.

This patch adds a new Process subsystem for running external programs in
the background of Tor. The design is focused around a new type named
`process_t` which have an API that allows the developer to easily write
code that interacts with the given child process. These interactions
includes:

- Easy API for writing output to the child process's standard input
  handle.
- Receive callbacks whenever the child has output on either its standard
  output or standard error handles.
- Receive callback when the child process terminates.

We also support two different "protocols" for handling output from the
child process. The default protocol is the "line" protocol where the
process output callbacks will be invoked only when there is complete
lines (either "\r\n" or "\n" terminated). We also support the "raw"
protocol where the read callbacks will get whatever the operating system
delivered to us in a single read operation.

This patch does not include any operating system backends, but the Unix
and Windows backends will be included in separate commits.

See: https://bugs.torproject.org/28179