Update mlsys_mailer_data.include include segment for Mutlics networking support

[Eric Swenson]

In order to support inter-system mail (e.g. over the Chaosnet or Internet), the include segment mlsys_mailer_data.incl.pl1 needs some additional flag bits:

Inserted in B:
B17                  4 returning bit (1) unaligned,                   /* now attempting to return to sender */
B18                  4 unspooling bit (1) unaligned,                  /* now attempting to deliver this message */
B19                  4 mbz1 bit (28) unaligned,
Preceding:
A17                3 source varying char (256),                       /* original source of mail */