Hide struct number_set implementation details from users

Hide struct number_set implementation details from users

* number_set.h (number_slot_t, struct number_set): Move to number_set.c.
(struct number_set): Add forward declaration.
(read_set, write_set, signal_set): Change prototypes from objects
to pointers.
* filter_qualify.c (read_set, write_set, signal_set): Change definitions
from objects to pointers.
(abbrev_set, inject_set, raw_set, trace_set, verbose_set): Change
definitions from arrays to pointers.
(qualify_read): Initialize read_set before first use.
(qualify_write): Initialize write_set before first use.
(qualify_signals): Initialize signal_set before first use.
(qualify_trace): Initialize trace_set before first use.
(qualify_abbrev): Initialize abbrev_set before first use.
(qualify_verbose): Initialize verbose_set before first use.
(qualify_raw): Initialize raw_set before first use.
(qualify_inject_common): Initialize inject_set before first use.
* strace.c (print_signalled, print_stopped): Update signal_set usage.
* syscall.c (dumpio): Update usage of read_set and write_set.