tunneling: Add df_default and df_inherit tunnel options.

tunneling: Add df_default and df_inherit tunnel options.

Split existing pmtud tunnel option's functionality into three. Existing pmtud
option still exists, but now governs only whether datapath sends ICMP frag
needed messages. New df_inherit option controls whether DF bit is copied from
packet inner header to outer tunnel header. New df_default option controls
whether DF bit is set if inner packet isn't IP or if df_inherit is disabled.

Suggested-by: Jesse Gross <jesse@nicira.com>
Signed-off-by: Andrew Evans <aevans@nicira.com>
Feature #5456.