Refactored usage of njs_ret_t.

Refactored usage of njs_ret_t.

Currently njs_ret_t is used in 2 different cases: as a jump offset for
bytecode and as a return value for ordinary functions. The second case
is quite similar with njs_int_t (and is often confused with).

1) Splitting this two cases into different types to avoid
confusion with njs_int_t.

2) Renaming njs_ret_t to njs_jump_off_t to better reflect its
purpose.

NO functional changes.