box: straighten log configuration code

See [1] for some details on why the code for log configuration needs
some care. In short log config validity checks are spread thru many
places, on some code paths we use one checks and on other code paths
other checks. And we make repetitive validity checks many times in
runtime on single configure call.

We can also reuse code for setting default values, checking options
type and resetting values to default.

- As a side effect of refactoring one can now reset values to default thru
  `log.cfg()` so now `log.cfg()` is on par with `box.cfg` in this respect.

- This patch also drops conversion `log_level` from string to number.

  Before (shorten):

    tarantool> box.cfg{log_level='warn'}
    tarantool> box.cfg.log_level
    - info
    tarantool> log.cfg.level
    - 5

  Also:
    tarantool> log.cfg{level='info'}
    tarantool> log.cfg.level
    - 5
    tarantool> box.cfg{}
    tarantool> box.cfg.log_level
    - 5

  After patch if `log_level`/`level` is given as string than it is saved
  and returned as string too. I guess it should not affect users but looks
  more handy.

- Also fixed issue with inconsistent setting `log_nonblock` thru
  `box.cfg()` and `log.cfg()`. In former case `nil` means setting default
  depending on logger type. In the latter case `nil` meant setting
  `nonblock` to `false`.

- Also patch fixes #7447.

Closes #7447.

[1] PR for this refactoring
https://github.com/tarantool/tarantool/pull/7454

NO_DOC=refactoring/tiny API improvemnent