timeval: Check for HAVE_BACKTRACE instead of HAVE_EXECINFO_H.

Other code in the tree uses HAVE_BACKTRACE and then blindly includes
<execinfo.h> if it is present, so this doesn't make anything worse.

Once we do that, HAVE_EXECINFO_H has no further users, so this commit also
removes the check for <execinfo.h>

Reported-by: YAMAMOTO Takashi <yamt@mwd.biglobe.ne.jp>
Signed-off-by: Ben Pfaff <blp@nicira.com>
FAQ: Describe how to connect bridges with patch ports.

I keep seeing this question.

Signed-off-by: Ben Pfaff <blp@nicira.com>
match: Correct formatting of MPLS

1. mpls or mplsm should be included in the formatted match  instead of
   dl_type=0xXXXX if the dl_type of the match is ETH_TYPE_MPLS or
   ETH_TYPE_MPLS_MCAST respectively. This is consistent with the treatment
   of other dl_types in match_format().

2. Add formatting of MPLS TTL to match_format().
   The format is mpls_ttl=YY.

3. Set the mpls_lse mask of a match to all 1's if
   the dl_type of the flow is ETH_TYPE_MPLS or ETH_TYPE_MPLS_MCAST.
   This will cause mpls_label=LLLL,mpls_tc=W,mpls_ttl=YY,mpls_bos=Z to
   be included with the match is formatted.

4. mpls(label:LLLL,tc:W,ttl:TT,bos:Z) and
   mplsm(label:LLLL,tc:W,ttl:TT,bos:Z) should not be included
   in a formatted match as this is not consistent with the
   formatting of other match elements.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>
FAQ: Explain why VMs on a VLAN can't access the Internet.

Signed-off-by: Ben Pfaff <blp@nicira.com>