net: sntp: Add option for SNTP uncertainty

SNTP response is not analyzed for uncertainty, and no uncertainty is given
to the `struct sntp_time` returned. Fix it with a Kconfig option that adds
optional SNTP uncertainty and timestamp fields in SNTP time struct, and
calculates these when parsing the response.
Adds two helper functions to convert Q16.16/Q32.32 in seconds to `int64_t`
in microseconds to facilitate this.
Also changes combined `lvm` field in `struct sntp_pkt` to bit-fields
`li`, `vn`, and `mode`.

Signed-off-by: Jasper Smit <git@jrhrsmit.nl>