e1d: reconnect to osmo-e1d after connection loss

When osmo-e1d crashes while a line is in use the client process may
experience not only a lost connection, it may also hang up in an endless
loop that would also spam the logfile. The reason for this is that the
e1d driver in libosmo-abis lacks mechanisms to detect when the
connection to osmo-e1d gets lost.

When osmo-e1d goes down the effects should be similar to those of a
normal connection loss of an e1 line (cable pulled). So let's add an FSM
that takes care of the recovery of the connection when it is lost. Also
make sure that no havoc happens when the connection gets lost.

Related: OS#5983
Change-Id: Iaf4d42c2f009b1d7666e319fabdeb2598aa0b338
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M src/input/e1d.c
1 file changed, 271 insertions(+), 15 deletions(-)