This makes use of the `periph_timer_query_freqs` feature:
1. It does choose the closest frequency supported before calling
timer_init() in the ztimer_periph_timer backend.
2. It does make use of the actually chosen frequency when using
`ztimer_convert_frac`.
3. It does `assert()` the frequency is within 5% of the specified when
no frequency conversion is performed or `ztimer_convert_shift_up`
is used.
To avoid confusions between `auto_init_ztimer` and `ztimer_auto_init`
rename `ztimer_auto_init` to `ztimer_init`.
This allows for similar handling as `saul_init_devs` and
`auto_init_devs`. Dependencies are therefore done against the USEMODULE
and not DEFAULT_MODULE or checking DISABLE_MODULE. For this, both
`ztimer_init` and `auto_init_ztimer` are added to DEFAULT_MODULE and
both need disabling if the user does not want that module in.
With this, the comment in Kconfig no longer applies.
