readelf(1) -S: Include zero index and match binutils' no-name

Include the SHN_UNDEF (zero) index special section in extended-attribute
ELF files, like GNU binutils' readelf.

Additionally, print "<no-name>" for sections without names, like GNU
binutils.

Reviewed by: kaiw@
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8707