xapcmd: use popen_rd for running xapian-compact

xapcmd: use popen_rd for running xapian-compact

public-inbox-compact wrapper displays progress by default,
anyways, and there's not a lot of output, so simplify our
code by using popen_rd instead of spawn + optional pipe.

While we're at it use "while (<HANDLE>)" to display
progress as it happens, since "foreach (<$HANDLE>)"
slurps the contents into an array, first.