strings: fix exit status if a file before the last one fails

Previously a command like "strings f1 f2 f3" reported the exit status
based only on processing the last file.

As with GNU strings, report an error exit status if an error was
encountered processing any of the files. While here simplify the
exit status handling to just success (0) / failure (1).

Reviewed by: brooks
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8334