Follow up to roundup feature addition in r289203

• Rename -r to -R to avoid the clash with makefs -r in NetBSD
• Note that -R is an FFS-specific option because it's not implemented in cd9660 today
• Rename the roundup variable to "roundup-size" in the manpage and help text for consistency with other variables.
• Bump .Dd (missed in r289203)

PR: 203707
MFC after: 1 week
X-MFC with: r289203
Differential Revision: https://reviews.freebsd.org/D3959
Reviewed by: adrian (earlier patch), emaste
Sponsored by: EMC / Isilon Storage Division