lib/process: Allow process output to be retrieved on each event

lib/process: Allow process output to be retrieved on each event

Allow a custom callback function to be set when registering the IO
waiter for asynchronous processes.

To allow output from processes to be parsed as it is received, add
process_stdout_custom() which passes a new "line" parameter to
process_read_stdout_once() in order to consume output as it appears.

Users of a custom IO callback will only have access to the process_info
struct which is internal to lib/process; the function
procinfo_get_process() is added to allow these callers to access process
information.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>