>        you can change FD_SETSIZE without libc shlib major bump.

No, in fact we cannot, as libc has some on-stack fd-set masks sized
for the *default* FD_SETSIZE, which are exposed further up the API and
ABI.  Increasing the default FD_SETSIZE will change that ABI in a
non-backward compatible way.

(If memory serves, the affected routines are somewhwere in the rpc
code; try "man 3 rpc" looking for fd_set, maybe svc_fdset.  We went over
this in detail some years back; you could try checking the archives.)