input: Make keep alive configurable (generic)

This patch adds a generic keep alive configuration layer that mainly
consists of additional fields in e1_input structs and VTY commands
and extensions.

Ticket: OW#1060
Sponsored-by: On-Waves ehf
diff --git a/TODO-RELEASE b/TODO-RELEASE
new file mode 100644
index 0000000..98b9a64
--- /dev/null
+++ b/TODO-RELEASE
@@ -0,0 +1,2 @@
+#library	what		description / commit summary line
+libosmoabis	abi-change	input: Make keep alive configurable (generic)