debian: Better document how to get a kernel module in README.Debian.

This moves existing text from INSTALL.Debian into the README.Debian
installed as part of the openvswitch-switch package, and adds a reference
from the former to the latter.

Reported-by: Brian Candler <b.candler@pobox.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Gurucharan Shetty <gshetty@nicira.com>
Clarify tunnel wildcarding.

It would seem we should set the 'tunnel.ip_dst' in 'wc' when calling
tnl_port_should_receive(), as it is reading that flow field.  However,
since tunnels' datapath port numbers are different from the non-tunnel
ports, and we always unwildcard the 'in_port', we do not need to
unwildcard the 'tunnel.ip_dst' for non-tunneled packets.

Also, 'nw_tos' need not be unwildcarded if it is not examined.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Makefile.am: Fix build hang when .git and Git are present but don't work.

If the .git directory is there, and "git --version" works, but "git
ls-files" fails to print any file names (e.g. perhaps because .git is a
symlink to an inaccessible directory) then the "grep" command in the
thread-safety-check rule would hang forever because it had an empty list
of files and therefore expected to read from stdin.  This fixes the problem
by adding the name of an empty file to the command, which is harmless.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>