vty should complain about unknown pchan in config file
If I misspell or freely invent PCHAN names in the osmo-nitb.cfg, osmo-nitb happily starts up
and all we get is a value error upon OML channel init.
The VTY should parse the pchan names and make sure only known channel names are used in the config file,
or reject startup with a parse error.
side note: it makes sense to abort the program startup, but not to abort a running osmo-bsc from the interactive telnet VTY.
It is possible to find out whether the command came from an interactive VTY or from initial config file:
if (vty->type == VTY_FILE) OSMO_ASSERT(valid_string);