Auto-detect TLS MinVersion integer base

[SaveTheRbtz]

From strconv.ParseInt:

If the base argument is 0, the true base is implied by the string's prefix following the sign (if present): 2 for "0b", 8 for "0" or "0o", 16 for "0x", and 10 otherwise. Also, for argument base 0 only, underscore characters are permitted as defined by the Go syntax for integer literals.

This is useful for example for hex constants from the crypto/tls, e.g.:

VersionTLS13 = 0x0304

PS. Codepath in the MinVersion parsing looks quite fragile.
PPS. Code in Min/Max should likely be the same.