stress2: Use the proper type for the getopt(3) return value

On arm platforms "char" is typically unsigned, in which case the test
"(c = getopt(...)) != -1" does not behave as desired.

Reviewed by: pho
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45365

stress2: Tune to get more slightly broken file systems

stress2: Occasionally modify 8 bits

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

stress2: Fix a off-by-one error. Added check for argument file type

stress2: Added an option to set the file size. Added missing error checks

stress2: Initial import

Discussed with: kib