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.

cp.1

@@ -27,7 +27,8 @@ has to be a directory.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl a
-Preserve block devices, character devices, sockets and FIFOs. Implies
+Preserve block devices, character devices, sockets and FIFOs.
+Implies
 .Fl p ,
 .Fl P
 and
@@ -55,8 +56,8 @@ 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.
+Traverse directories recursively.
+If this flag is not specified, directories are not copied.
 .El
 .Sh SEE ALSO
 .Xr mv 1