e1_input: Allow to change the pcap fd and/or unset it
e1_input: Allow to change the pcap fd and/or unset it

This will update the pcap fd in all open lines and close
the previous one (if applicable).

Signed-off-by: Sylvain Munaut <349f318521b45672c77207abf1a5ed60eb011db1@246tNt.com>
Change-Id: I5c7dd740ba0a90b40c69a53b3dcc9d6d6a98f660

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis