ln: Add support for target directories

Also, now that we are using {sym,}linkat, implement the trivial -L and
-P options.

ln.1

@@ -3,12 +3,12 @@
 ln \- make links between files
 .SH SYNOPSIS
 .B ln
-.RB [ \-fs ]
+.RB [ \-LPfs ]
 .I file
 .RI [ name ]
 .P
 .B ln
-.RB [ \-fs ]
+.RB [ \-LPfs ]
 .RI [ file ...]
 .RI [ directory ]
 .SH DESCRIPTION
@@ -18,6 +18,13 @@ it is linked into the current directory.  If multiple files are listed they will
 be linked into the given directory.
 .SH OPTIONS
 .TP
+.B \-L
+create links to the files referenced by symbolic link source files (default
+behavior).
+.TP
+.B \-P
+create links to symbolic link source files themselves.
+.TP
 .B \-f
 remove existing destinations.
 .TP