fsck: Make -Cn work correctly where n != 0

fsck: Make -Cn work correctly where n != 0

Previously, fsck was only passing in -Cn to the first e2fsck process
to start up, and enabling the progress information by sending a
SIGUSR1 signal.  This didn't work if the progress information was
intended to go to file descriptor, since the information was never
passed to e2fsck.

So we now pass the progress fd in as a negative number if the progress
information is intended to be initially suppressed.

Addresses-Launchpad-Bug: #203323
Addresses-Sourceforge-Bug: #1926023

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>