This patch do --thread option optional (according to docs).
Currently it is marked as "mandatory" option and it conflicts with official docs.
Also this patch includes following changes:
- add test for -data-disassemble
- add tests for -exec-next/-exec-next-instruction/-exec-step/-exec-step-instruction/-exec-finish commands
- improve test program (a.c, b.c, main.c)
- fix TestMiBreakpoint.py and TestMiSyntax.py (it's required after previous item)
- add tests for -stack-info-depth and -stack-list-frames