maint: omit function defn extern

maint: omit function defn extern

* src/chown-core.c, src/copy.c, src/cp-hash.c, src/csplit.c:
* src/expand-common.c, src/find-mount-point.c, src/force-link.c:
* src/group-list.c, src/iopoll.c, src/operand2sig.c:
* src/show-date.c, src/wc_avx2.c:
Omit unnecessary ‘extern ’ at the start of function defns.
This is less wordy, makes it a bit easier to grep for issues such
as the missing consistency checking in cksum.