add some mandoc pages and fix some warnings

- 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.

chmod.1

@@ -1,10 +1,11 @@
 .Dd January 17, 2015
 .Dt CHMOD 1 sbase\-VERSION
+.Os
 .Sh NAME
 .Nm chmod
 .Nd change file mode
 .Sh SYNOPSIS
-.Nm chmod
+.Nm
 .Op Fl R
 .Ar mode
 .Op Ar file ...