flock: zero timeout is valid

This patch reverts Sami's "timeout cannot be zero", introduced
in commit 605325b23b36238c8f3ae165e37cab9064553cf7.

The --timeout 0 has been originally interpreted as --nonblock. The
patch also add hint about this behavior to the man page.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1149974
Signed-off-by: Karel Zak <kzak@redhat.com>