cve-update-nvd2-native: new CVE database fetcher

Add new fetcher for the NVD database using the 2.0 API [1].
The implementation changes as little as possible, keeping the current
database format (but using a different database file for the transition
period), with a notable exception of not using the META table.

Minor changes that could be visible:
- the database starts in 1999 instead of 2002
- the complete fetch is longer (30 minutes typically)

[1] https://nvd.nist.gov/developers/vulnerabilities



Signed-off-by: Marta Rybczynska <marta.rybczynska@syslinbit.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>