Rename variables hiding others defined on another scope.

Rename variables hiding others defined on another scope.

* gdbtk/generic/gdbtk-cmds.c (gdb_find_file_command): rename struct stat st to
  sbuf.
* gdbtk/generic/gdbtk-varobj.c (variable_update): rename Tcl_Obj *var to v.
* gdbtk/generic/gdbtk.c: global gdbtk_source_filename is now an std::string.
  (gdbtk_source_start_file): drop const char *s, rephrase concatenation in C++.
  (gdbtk_test): xstrdup is not needed anymore.