du: Print progress information to stderr

• On SIGINFO, print the current path to stderr rather than stdout.

• Do so immediately, instead of the next time we finish a directory.

• Document this behavior in the manual page.

PR: 296861
MFC after: 1 week
Fixes: d1588599c024 ("Report the next directory being scanned ...")
Reviewed by: wollman
Differential Revision: https://reviews.freebsd.org/D58702