ovs-vsctl: Add --if-exists option to many database commands.

A few ovs-vsctl commands have accepted --if-exists options for some time,
to make it possible to execute them in cases where it doesn't really
matter if the records they touch exist.  This commit adds this option to
other commands.

This is intended for initial use with "ovs-vsctl set interface <iface>
ofport_request=<number>" commands in ovs-ctl for upgrades from OVS 1.9
to later versions.

This makes a very small incompatible change to the ovs-vsctl "get"
command: previously with --if-exists, only keys in map columns were
allowed to be missing; now with --if-eixsts, also records are allowed
to be missing.

CC: Gurucharan Shetty <gshetty@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>