msgb: add msgb_talloc_ctx_init(), deprecate msgb_set_talloc_ctx()

So far each and every main() scope creates a msgb talloc context and either
passes it to msgb_set_talloc_ctx() or sets tall_msgb_ctx directly (by defining
it extern first).

Remove some code duplication: add one central function that creates the "msgb"
talloc context for all.

Most users of msgb employ a talloc_named_const(), but osmo-bts uses a
talloc_pool() instead. Offer both ways by means of the pool_size argument, and
for both ways make sure the context is called "msgb".

Suggest that msgb users should move to this new function: deprecate
msgb_set_talloc_ctx(). To be able to do so, include core/defs.h in msgb.h.

There's a tradeoff between hiding the msgb talloc context behind API that tries
to guess all use cases versus avoiding code dup. This patch opts against code
dup and boldly assumes that all future use is covered.

Also, the new function suggests to not access tall_msgb_ctx directly, which can
be considered a style improvement.

It seems that not all main scopes that use msgb actually initialize the msgb
ctx. As a fallback for these, explicitly initialize tall_msgb_ctx to NULL.

Change-Id: I747fbbf977c4d2c868c8dead64cfc5fd86eb8d4c

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