This part XII of cmd engine rewrite.
This serie implements proper support for && and || command chaining operators: - implement proper precedence parsing - implement semantics for the operators - for this to work, we need to have expected success/failure from commands; this is supported here for external commands (and calling into a label or another BAT/CMD file), but support has to be added for all builtin commands - that's a big mess as behaviors is far from homogeneous among the builtins: + some builtin set coherent success/failure vs errorlevel information + some report success/failure, but only set errorlevel upon failure (keeping errorlevel from previous commands untouched), + some even report success (even in case of failure) yet set errorlevel in case of failure, - so the next series will now concentrate on testing success/failure and errorlevel for every builtin command (sigh) - for now, to help the transition phase, we can distinguish if a builtin properly reports success/failure; if a builtin with success/failure ability is used in a chaining operator, we implement the intented behavior of the chaining operator; otherwise, we keep old behavior (ie always execute LHS and RHS whatever the success/failure status). This shall be removed when all builtin commands report success/failure.