Changes

pcap-server: remove unneeded check for null base_path

It cannot be ever NULL. It is set to a non-null path during alloc(), and
it can only be replaced with another string through VTY.

Change-Id: I2f503f6f9af35d6ae6fbd03e85cade4f0eb93df2

pcap-server: Resolve real path of pcap file before opening

This means we always end up with a canonicalized absolute path.

While at it, also valdiate that we can indeed resolve the path (eg. it
exists and is reachable with current process permissions) during
startup.

It is preferrable to resolve it everytime a file is opened, this allows
the user to eg. change the base-path to a different symlink it they wish
to change the base-path for new pcaps without restarting osmo-pcap.

Change-Id: I8d161010dc8b480dd4cf90e19ca28a77914a50ad

pcap-server: Introduce VTY cmd '[no] completed-path'

This VTY allows enabling feature to instruct osmo-pcap-server to move
the closed files (due to ration) into a separate directory.
This is useful for users who want to monitor and act on closed pcap
files to eg. generate statistics, etc.

Important: completed-path must be in the same filesystem mountpoint as
base-path, since osmo-pcap-server is actually not copying the files,
but atomically renaming the paths through rename() syscall.

Related: SYS#7248
Change-Id: I5166d1c5d9eb45358c87c2e1c5fbf7969d1d5294