* compiler: -g and -debug should always add -g at least in the C compiler by default.
* Set first the C backend debug_options, so that the optimization_options can override them later.
* Implement cleaner named is_debug, is_vlines, is_keep_c and is_cache options (i.e. -g/-cg, -keep_c, -cache v flags).
* Check and reset file/line numbers in the generated main program, only if v.pref.is_vlines is true.
* v test: support for running 'v test folder/' .
* Support passing multiple folders and also single _test.v files to 'v test' .
* Update vhelp too, with descriptions of v test folder/ and v -stats .
* Fix running `v test v` from outside the root of the v tree.
