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.

mktemp.1

@@ -13,7 +13,8 @@
 .Nm
 creates a temporary file by generating a unique filename with
 .Ar template ,
-which has to have at least six 'X's appended to it.  If no
+which has to have at least six 'X's appended to it.
+If no
 .Ar template
 is specified, a default of 'tmp.XXXXXXXXXX' is used and the
 tmpdir set to '/tmp' or, if set, the TMPDIR environment variable.
@@ -24,10 +25,11 @@ Create a temporary directory instead of a file.
 .It Fl p Ar directory
 Use the specified
 .Ar directory
-as a prefix when generating the temporary filename. The directory will be
-overridden by the user's
+as a prefix when generating the temporary filename.
+The directory will be overridden by the user's
 .Ev TMPDIR
-environment variable if it is set. This option implies the
+environment variable if it is set.
+This option implies the
 .Fl t
 flag (see below).
 .It Fl q
@@ -37,9 +39,11 @@ Generate a path rooted in a temporary directory.
 .It Fl u
 Unlink file before
 .Nm
-exits. This is slightly better than
+exits.
+This is slightly better than
 .Xr mktemp 3
-but still introduces a race condition. Use of this option is not encouraged.
+but still introduces a race condition.
+Use of this option is not encouraged.
 .El
 .Sh SEE ALSO
 .Xr mkdtemp 3 ,