[RFE] add new "kernel log buffer" target, or at least a functions to feed qb_log_custom_open with that effect

It would make a tonne of sense if logging clients running on Linux had
a convenient ability to opt-in to log urgent messages also to `kmsg`,
since it may be at times backed with actual firmware-specific persistent
storage[1,2] that may obviate various disk synchronization issues,
which may othetwise prevent some before-death messages to be made
durable at all.

Alternative approach could be to utilize persistent storage directly
by the client program, e.g. by dumping a blackbox file there, but
e.g., in case of kernel crash, it is not achievable.

[1] brief notes with links:
https://github.com/abrt/abrt/wiki/pstore-oops
[2] https://lwn.net/Articles/434821/

Start by reading the qb_log_custom_open API and the issue's references on Linux kmsg and persistent storage. Determine how an opt-in kernel log buffer target or equivalent function would fit the existing logging interface, then verify that urgent messages can reach kmsg without removing current destinations.

Written by the indexing model from the issue text.