readelf: Actually dump hex or strings when -p or -x get section number.

readelf: Actually dump hex or strings when -p or -x get section number.

The readelf code did parse section numbers, but then failed to actually
dump the section found. Fixed by actually calling the dump function
(either the hex or string variant). Add testcase for readelf -x num.

Signed-off-by: Mark Wielaard <mark@klomp.org>