cp: Default to -P when -R is specified

POSIX only specifies the -H, -L, and -P options for use with -R, and
the default is left to the implementation. Without -R, symlinks must
be followed.

Most implementations use -P as the default with -R, which makes sense
to me as default behavior (the source and destination trees are the same).

Since we use the same code for -R and without it, and we allow -H, -L,
and -P without -R, set the default based on the presence of -R. Without
it, use -L as before, as required by POSIX, and with it, use -P to match
the behavior of other implementations.

cp.1

@@ -48,9 +48,12 @@ Dereference
 if it is a symbolic link.
 .It Fl L
 Dereference all symbolic links.
-This is the default.
+This is the default without
+.Fl R .
 .It Fl P
 Preserve symbolic links.
+This is the default with
+.Fl R .
 .It Fl R
 Traverse directories recursively. If this flag is not specified, directories
 are not copied.