lib: Increase HIVEX_MAX_VALUE_LEN to 8000000.

I encountered a hive that contained a key of 3_886_561 bytes length in
a key called 'HKLM\SYSTEM\ControlSet001\services\mfeavfk' (apparently
belonging to "McAfee Anti-virus software").

The previous limit was set arbitrarily at 2_000_000 bytes.  Increase
it to cope with this larger key.