lib/process: Add process_get_stdout

lib/process: Add process_get_stdout

Add a new structure 'struct process_stdout' and optional parameter
'stdout' to the process_run_simple functions to allow the caller
to get a buffer filled with the stdout from the child process.

Rename the process_run_simple functions to process_get_stdout
and add wrappers for the old process_run_simple function names.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>