maint: use MAYBE_UNUSED more consistently

maint: use MAYBE_UNUSED more consistently

* src/basenc.c (no_required_padding):
* gl/lib/smack.h (smack_new_label_from_self)
(smack_set_label_for_self):
* src/cksum.c (output_crc):
* src/digest.c (md5_sum_stream, sha1_sum_stream)
(sha224_sum_stream, sha256_sum_stream, sha384_sum_stream)
(sha512_sum_stream, sm3_sum_stream, output_file):
* src/stat.c (print_statfs, print_stat):
* src/sum.c (output_bsd, output_sysv):
* src/timeout.c (chld):
Mark possibly-unused parameters with MAYBE_UNUSED.