Keep track of the full registry path (including the root key) in
currentKeyName. This makes the error messages more informative.
Reduce the number of global variables used by openKey() & co.  Only
use one variable, currentKeyHandle, to indicate whether a key is
currently open or not.

The only place they are allowed is in .reg files, so exclusively deal with them in processRegEntry().
Fix detection of the trailing ']' in processRegEntry(): it can be followed by some garbage.

Don't complain if the value already does not exist.

Make sure it does not modify the value if the data is invalid.