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
1) Add missing parens around assignment that is compared to zero.
2) Make some variables that only take non-negative values unsigned.
3) Some casts/type changes to fix other constness warnings.
4) Make one variable a const char *.
5) Make sure termwidth is positive, it doesn't make sense for it to be negative.
Approved by: dds
