sed(1): Don't force a newline on last line, if input stream doesn't have one

While here, change how we check if the current line is the last one.
Before, we just checked if there were more files after the current one.
Now, we check the actual content of those files: they files may not have
a line at all. This matches the definition of the "last line" by the
Open Group.

The new behavior is closer to GNU sed.

PR:		160745
Phabric:	https://phabric.freebsd.org/D431
Reviewed by:	jilles
Approved by:	jilles
Exp-run by:	antoine