hivex_root: Return errno == HIVEX_NO_KEY when root key is missing.

hivex_root: Return errno == HIVEX_NO_KEY when root key is missing.

Previously we returned errno == ENOKEY.  However this was an
unfortunate choice of error code since it is not defined in POSIX.  As
a result it is missing on several platforms.

HIVEX_NO_KEY is defined as ENOKEY on platforms where this symbol
exists (thus maintaining backwards ABI compatibility), and defined as
another POSIX error code otherwise.