Make the r command filename obligatory, similar to what FreeBSD and NetBSD

Make the r command filename obligatory, similar to what FreeBSD and NetBSD
do for several years.
While here make corresponding error message for missing read and write file
consistent between commands/flag, and shrink the the code of the w flag of
the s command by making it use the same code as the w command.

Prompted by a larger diff by kshe59 <at> zoho <dot> eu
OK millert@