Fix various lint warnings in manuals

Fix "new sentence, new line" warnings throughout so that formatters
can produce correct spacing between sentences.

join.1
  Remove unnecessary Ns macros. These are not necessary for delimeters,
  which get special treatment.

xinstall.1
  Fix date in manual. The contents were last modified on 2016-12-03,
  so use that instead of the invalid date.

grep.1
  Fix escape sequence for `\<` and `\>`.

ed.1
  Remove spurious `\\n` escape for the null-command.

du.1

@@ -18,7 +18,8 @@
 displays the file system block usage for each
 .Ar file
 argument and for each directory in the file hierarchy rooted in directory
-argument. If no
+argument.
+If no
 .Ar file
 is specified, the block usage of the hierarchy rooted in the current directory
 is displayed.
@@ -43,7 +44,8 @@ recursively traversing directories.
 .It Fl L
 Always dereference symbolic links while recursively traversing directories.
 .It Fl P
-Don't dereference symbolic links. This is the default.
+Don't dereference symbolic links.
+This is the default.
 .It Fl x
 Do not traverse file systems mount points.
 .El