layer23: Support configuring GSMTAP through VTY in l23 apps.

This allow all l23 apps supporting L23_OPT_VTY and L23_OPT_TAP to
dynamically configure gsmtap through a "gsmtap" VTY node:

gsmtap
 remote-host 127.0.0.1
 lchan sacch
 lchan lsacch
 lchan sacch/4
 lchan sacch/8
 lchan sacch/f
 lchan sacch/h
 lchan unknown
 lchan bcch
 lchan ccch
 lchan rach
 lchan agch
 lchan pch
 lchan sdcch
 lchan sdcch/4
 lchan sdcch/8
 lchan facch/f
 lchan facch/h
 lchan pacch
 lchan cbch
 lchan pdch
 lchan pttch
 lchan cbch
 lchan tch/f
 lchan tch/h
 category gprs dl-unknown
 category gprs dl-dummy
 category gprs dl-ctrl
 category gprs dl-data-gprs
 category gprs dl-data-egprs
 category gprs ul-unknown
 category gprs ul-dummy
 category gprs ul-ctrl
 category gprs ul-data-gprs
 category gprs ul-data-egprs

VTY cmd "lchan" enables/disables tapping on general channel types,
while "category" allows fine-grained selection of messages being
tapped within the enabled lchan which would tap those messages.

Change-Id: I2582a1633d37d350a7f4c2bb5e03793bdf46e839
---
M src/host/layer23/include/osmocom/bb/common/l23_app.h
M src/host/layer23/include/osmocom/bb/common/vty.h
M src/host/layer23/src/common/l1ctl.c
M src/host/layer23/src/common/main.c
M src/host/layer23/src/common/sim.c
M src/host/layer23/src/common/vty.c
M src/host/layer23/src/mobile/main.c
7 files changed, 337 insertions(+), 20 deletions(-)