veth: Handle flags correctry

Flags for a peer override flags for the other and not used for the
peer.

before:
# ip link add up type veth peer down multicast off
# ip link
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: veth0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 2e:5c:cd:f5:63:d2 brd ff:ff:ff:ff:ff:ff
3: veth1: <BROADCAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 72:b0:fa:1e:76:7a brd ff:ff:ff:ff:ff:ff

after:
# ip link add up type veth peer down multicast off
# ip link
1: lo: <LOOPBACK> mtu 65536 qdisc noop state DOWN mode DEFAULT group default
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: veth0: <BROADCAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000
    link/ether 6e:db:03:b3:bd:ff brd ff:ff:ff:ff:ff:ff
3: veth1: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000
    link/ether a6:62:d9:84:f0:73 brd ff:ff:ff:ff:ff:ff

Signed-off-by: Kusanagi Kouichi <slash@ac.auone-net.jp>
ipaddress: do not add IFA_FLAGS when not necessary

commit 37c9b94ed21d5779acc23d89a4 (add support for extended ifa_flags)
introduced a regression:

        # ./ip/ip addr add 192.168.0.1/24 dev eth0
        RTNETLINK answers: Invalid argument

This is due to old kernels don't support IFA_FLAGS flag, we should not
use it if we don't use the flags beyond old .ifa_flags.

Cc: Jiri Pirko <jiri@resnulli.us>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
ip: officially support flag mngtmpaddr also for "ip addr del"

Kernel is being extended to support flag IFA_F_MANAGETEMPADDR also for
deletion of addresses. This will allow a userspace application to indicate
that for a global address the kernel should delete all related temporary
addresses as well.

"ip addr del" internally calls ipaddr_modify which silently accepts
any flag provided on the command line already, independent of the
actual command.
Therefore only the usage documentation needs to be extended.

Signed-off-by: Heiner Kallweit <heiner.kallweit@web.de>
v3.14.0