rangesets: use PCTRIE_DEFINE

subr_rangeset.c is the only source file that calls functions like
pctrie_insert and pctrie_remove directly; other users of pctries use
the PCTRIE_DEFINE macro to define interfaces to pctrie that let them
ignore issues of offsets within structs and uint64_t return values.

Change subr_rangeset.c to use PCTRIE_DEFINE too. And change pctrie.h
to mark the lookup function as unused, to avoid warnings when
compiling files, like subr_rangeset.c, that don't invoke lookup().

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D41391