sdt_query: initialize probe_type/loc as unknown.

sdt_query: initialize probe_type/loc as unknown.

The fields, probe_type and probe_loc, are meaningless to start, and are
later set as the queried module is examined.  Commit 74fe61bc tried to
make sure that all POD types in this class are initialized, but these
two were essentially self-assigned.  Gcc misses this uninitialized use,
but clang caught it:

  CXX    stap-tapsets.o
../tapsets.cxx:6058:38: error: field is uninitialized when used here [-Werror,-Wuninitialized]
  base_query(dw, params), probe_type(probe_type), probe_loc(probe_loc), base_probe(base_probe),
                                     ^
../tapsets.cxx:6058:61: error: field is uninitialized when used here [-Werror,-Wuninitialized]
  base_query(dw, params), probe_type(probe_type), probe_loc(probe_loc), base_probe(base_probe),
                                                            ^

Values for "unknown" are now added to these enum types to initialize
with, and dealt with where needed.