h/rcvmail.h: Delete. Contained just a couple of pointless macros.

RCV_MOK was 0, RCV_MBX was 1.  They were used for exit(3) and done().
`exit(0)' is clearer than another layer of abstraction, and the commands
that used them didn't seem to document the exit statuses anyway.  The
included by other means, at least on one platform.