Rename fields in lvm_property_type.

Rename fields in lvm_property_type.

Based on review comments, rename a few fields in lvm_property_type.
In particular, change 'is_writeable' to 'is_settable', which is
more intuitive to the intent of the bitfield (a 'set' function
exists for this field/property).  Also, remove the char array
for 'id' - unnecessary as we can just use the string passed in
to do the strcmp.  Finally rename the union members from n_val
to 'integer' and 's_val' to 'string'.