Add chown (set user, set user:group) and chgrp (set group) commands

[mlwege]

Add chown (set user, set user:group) and chgrp (set group) commands, please.

Windows has this capability, as demonstrated by SFU, Cygwin, MSYS2.

[mlwege]

Example:
Group owner of a file can be changed with this API:

DWORD dwResult = SetSecurityInfo(
        hFile,                       // Handle to the object
        SE_FILE_OBJECT,              // Type of object (File)
        GROUP_SECURITY_INFORMATION,  // Information type to set (Primary Group)
        NULL,                        // Owner SID (NULL = don't change)
        pSidGroup,                   // Group SID (The new group SID)
        NULL,                        // DACL (NULL = don't change)
        NULL                         // SACL (NULL = don't change)
    );