Changes

e1line_dump_vty(): dump keepalive state and params

There is currently no obvious way to know if the keepalive is enabled
and which parameters are in use.  Executing 'show running-config'
command in the VTY would not always reveal the current configuration,
because it tends to omit parameters with default values.

Let's print the keepalive state and params in the output of the
'show e1_line' command.  Below is a few examples:

! keepalive is disabled
OsmoBSC# show e1_line
E1 Line Number 0, Name , Driver ipa
Keepalive: disabled
IPA Keepalive: disabled

! TCP Keepalive is enabled (default)
OsmoBSC# show e1_line
E1 Line Number 0, Name , Driver ipa
Keepalive: enabled
  Number of probes: (driver's default)
  Idle timeout: (driver's default)
  Probe interval: (driver's default)
IPA Keepalive: disabled

! TCP and IPA keepalive enabled (custom params)
OsmoBSC# show e1_line
E1 Line Number 0, Name , Driver ipa
Keepalive: enabled
  Number of probes: 2
  Idle timeout: 1s
  Probe interval: 3s
IPA Keepalive: enabled
  Interval: 2s
  Timeout: 10s

Note that in the case of TCP keepalive with default parameters
we cannot retrieve the actual defaults because that would be a
layering violation.  Thus we say "(driver's default)".

Change-Id: I17bd991850333ee794ab216f474b5e045fb01fa3
Related: OS#6375, SYS#6801