New interface task_spawn(); exec_builtin() now uses task_spawn(); All argv...

New interface task_spawn(); exec_builtin() now uses task_spawn(); All argv types should be char * const * not const char **

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5598 42af7a65-404d-4744-a932-0658087f49c3