restored vadddomain/vdeldomain to use library functions rather than invoking

restored vadddomain/vdeldomain to use library functions rather than invoking
 vpopmail executables
implemented vaddaliasdomain natively
global variable to store whether vpopmail was used and only vclose() if
 necessary
return the functions to returning true/false for success/failure rather than
 some arbitrary vpopmail return value
return the functions to not doing everything at once as you then lose
 fine-grain control over its operation
change C++ comments to C