Allow multiple --network flags for podman run/create

[Luap99]

We allow a container to be connected to several cni networks
but only if they are listed comma sperated. This is not intuitive
for users especially since the flag parsing allows multiple string
flags but only would take the last value. see: spf13/pflag#72

Also get rid of the extra parsing logic for pods. The invalid options
are already handled by pkg/specgen.

A test is added to prevent a future regression.

[mheon]

I'm not strictly opposed, but if we do this we need to be very sure the manpage makes it clear that this is only for multiple networks - I'm afraid of users thinking that podman run --net slirp4netns --net host and similar invocations are valid.

[rhatdan]

Could we add a failure mode where the user did do --net=host --net=container:foobar

[TomSweeneyRedHat]

The code LGTM, but share @mheon's thought on man page updates for this.

[zhangguanzhang]

Could we add a failure mode where the user did do --net=host --net=container:foobar

agree

[Luap99]

Updated. This is now the error.

$ podman run --network host --network foo-a alpine
Error: network conflict between type host and bridge

I also removed the extra pod parsing logic. The invalid options are already handled in pkg/specgen

[mheon]

/approve
Restarted one test which looked flaked.
LGTM

[openshift-ci-robot]

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[Luap99]

Tests are green.
@containers/podman-maintainers PTAL

[TomSweeneyRedHat]

LGTM
Thanks @Luap99 !

[mheon]

/lgtm