(shp_maxno): Variable's value is well-known constant N_MAXSHIPS + 1.

(shp_maxno): Variable's value is well-known constant N_MAXSHIPS + 1.
Useless, remove.  Users changed to check mchr's sentinel, because
that's clearer than comparing the index to N_MAXSHIPS or sizeof(mchr)
/ sizeof(*mchr).

(mchr): Change sentinel's m_name to NULL.