* breakpoint.c: (watch_command_1): Parse additional optional

  * breakpoint.c: (watch_command_1): Parse additional optional
      "thread" parameter to the watchpoint command and set the
      "thread" member of the breakpoint struct.
      * doc/gdb.texinfo: Add new parameter's description.
      * testsuite/gdb.base/watch_thread_num.c: New testcase source file.
      * testsuite/gdb.base/watch_thread_num.exp: New testcase expect file.