MFC r267265: truncate: Detect integer overflow, fix relative sizes, add tests.

The change to expand_number (r204654) broke detection of too large sizes and
relative sizes ('+'/'-').

Also add some tests.

The usr.bin/truncate/Makefile is slightly different in the MFC because
src.opts.mk does not exist in stable/10.

PR: 190735
Submitted by: Kirk Russell