abis: Fix line leaked & recreated upon every reconnect
abis: Fix line leaked & recreated upon every reconnect

Previous code creating a new line was really a workaround to have it
working while previous lines were being stacked internally inside
libosmo-abis.
Let's handle reference counts for the line properly and keep using the
same line always (since it can be further configured by libosmo-abis' VTY).
Recent patches to libosmo-abis fixed a bug where e1inp_line_update()
would only work the first time it was called on a e1np_line, and follow
up calls would be no-ops. This fix allows re-configuring and hence
reusing the same line to reconnect to BSC.

Depends: libosmo-abis Change-Id: Iff8092f88726cf238dac2abb99e135ac5864272d
Change-Id: Id7fd3ef81bbc517821def31f1a60f905a0e2fb52

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts