1) Make argument-naming consistent with other tools (cp(1), ...)
2) style fixes
3) usage() fix
4) BUGFIX: Probably from the old non-arg.h days, the directory-
check was only done when argc > 3, but with arg.h, this ignores
the case when 3 arguments were given.
This is actually a pretty serious issue and I'm glad it's fixed.
5) Moreover, be more verbose when stat() fails and make it clearer
what the hell is going on at this checkpoint.
- add .Os, it is mandatory.
- don't redeclare .Nm when it's not needed.
- fix some warnings (checked with mandoc -Tlint).
- remove some leftover old stuff.
