POSIX 2013 audit

Mostly manpage-shuffling according to the changes in the corrigendum,
wording changes and more idiomatic expressions.
All this is finished up by marking the POSIX 2013 conformant tools
with
.St -p1003.1-2013
which is not available in older mandoc builds or nroff, but which
reflects what we actually did, so who cares?

This is a huge step and it's not far until we can release sbase 0.1.

cp.1

@@ -1,4 +1,4 @@
-.Dd March 2, 2015
+.Dd March 27, 2015
 .Dt CP 1
 .Os sbase
 .Sh NAME
@@ -61,11 +61,11 @@ are not copied.
 The
 .Nm
 utility is compliant with the
-.St -p1003.1-2008
+.St -p1003.1-2013
 specification except from the
 .Op Fl i
 flag.
 .Pp
-The flags
+The
 .Op Fl av
-are an extension to that specification.
+flags are an extension to that specification.