CTRL: get rate_ctr_group by the id from rate_ctr_group_set_name()

In CTRL, add keyword "by_id" to use the given name for group instance
lookup. IOW, in addition to currently:

  GET 123 rate_ctr.abs.hnb.0.iuh:established

also implement:

  GET 123 rate_ctr.abs.hnb.by_id.001-01-L2342-R0-S55-C1.iuh:established

Some objects always have fixed indexes, because they come from the
config file (think "bts.0" or "msc.0").

However, some object IDs are created dynamically, which makes it
"impossible" to verify them in tests. In osmo-hnbgw, the same hNodeB may
show up as hnb.0, hnb.1, etc.

We do already have the rate_ctr_group_set_name() API which allows
tagging an identifier on a rate counter group. For above example,
osmo-hnbgw sets rate_ctr_group_set_name(cell_id_str).

When merging this patch, magically all rate_ctr_groups in all osmo
programs will become accessible by their given rate_ctr_group_set_name()
IDs.

This also means that the strings passed to rate_ctr_group_set_name() now
are required to be a valid identifier, to not mix up the CTRL interface
syntax. So far, only log an error, to figure out if we need mangling.

Related: OS#6545
Change-Id: I841a12f76e6fcb2bd7aecb5f4b1707d9ec927137
---
M include/osmocom/core/rate_ctr.h
M src/core/libosmocore.map
M src/core/rate_ctr.c
M src/ctrl/control_if.c
4 files changed, 65 insertions(+), 15 deletions(-)