Make sector types signed

Make sector types signed

get_empobj_chr() and emp_obj_chr_name() access struct sctstr member
sct_type through struct empobj member type.  This is technically
non-portable, because the two differ in signedness.  It was also
undocumented.  Fix by making sct_type signed.  sct_newtype as well,
for consistency.

map_char() uses unsigned char for a sector type argument.  Change that
to int.  Matches how this is done elsewhere.