top(1): Quiesce several warnings

This is all warnings at level six (6) that are not
char-subscripts, incompatible-pointer-types,
sign-compare, switch, int-conversion,
missing-variable-declarations, cast-qual, cast-align

Some warnings that are fixed by this commit are:
shadow, strict-prototypes, missing-prototypes, pointer-arith,
unused-parameter, unused-const-variable, and several others