hivex: Add HIVEX_OPEN_WRITE flag to allow hive to be opened for writing.

hivex: Add HIVEX_OPEN_WRITE flag to allow hive to be opened for writing.

If this flag is omitted (as in the case for all existing callers)
then the hive is still opened read-only.

We add a 'writable' flag to the hive handle, and we change the way
that the hive file (data) is stored.  The data is still mmapped if
the file is opened read-only, since that is more efficient and allows
us to handle larger hives.  However if we need to write to the file
then we have to read it all into memory, since if we had to extend the
file we need to realloc that data.

Note the manpage section L</WRITING TO HIVE FILES> comes in a later
commit.