BUG: errors: remove printf positional args for user messages context

BUG: errors: remove printf positional args for user messages context

Change the algorithm for the generation of the user messages context
prefix. Remove the dubious API relying on optional printf positional
arguments. This may be non portable, and in fact the CI glibc crashes
with the following error when some arguments are not present in the
format string :

"invalid %N$ use detected".

Now, a fixed buffer attached to the context instance is allocated once
for the program lifetime. Then call repeatedly snprintf with the
optional arguments of context if present to build the context string.
The buffer is deallocated via a per-thread free handler.

This does not need to be backported.