analyze: Fix verify exit status regression
    
    Previously, systemd-analyze verify would return 0 even if warnings
    were raised during analysis of the specified units or their
    dependencies. With 3cc3dc7, verify was changed to return 1 when
    warnings were raised.
    
    This commit changes the default mode to _RECURSIVE_ERRORS_INVALID
    so that verify returns zero again by default when warnings are
    raised.