lib/process: Don't abort stdout reads on EINTR

lib/process: Don't abort stdout reads on EINTR

If our read() of the process stdout pipe fails with EINTR (eg, if we
receive a SIGCHLD because the process exited), then
process_read_stdout_once will return a non-zero exit code, and we'll
abort any further stdout collection.

Instead, we should check for EINTR, and allow the reads to continue.

This change normalises the return value from process_read_stdout_once to
return positive on success, negative on failure, and zero on competion.
We use a positive return value for the non-error EINTR case.

Also, add a pb_log if the read fails for non-EINTR reasons.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>