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.

od.1

@@ -16,7 +16,8 @@
 .Nm
 writes an octal dump of each
 .Ar file
-to stdout.  If no
+to stdout.
+If no
 .Ar file
 is given
 .Nm
@@ -27,7 +28,8 @@ reads from stdin.
 .Ar addressformat
 is one of d|o|x|n and sets the address to be
 either in \fId\fRecimal, \fIo\fRctal, he\fIx\fRadecimal or \fIn\fRot
-printed at all.  The default is octal.
+printed at all.
+The default is octal.
 .It Fl E | e
 Force Little Endian
 .Fl ( e )
@@ -59,7 +61,8 @@ he\fIx\fRadecimal format, processing the given amount of bytes or the length
 of \fIC\fRhar, \fIS\fRhort, \fII\fRnteger or \fIL\fRong.
 The default is octal with 4 bytes.
 .It Fl v
-Always set. Write all input data, including duplicate lines.
+Always set.
+Write all input data, including duplicate lines.
 .It Fl x
 Equivalent to
 .Fl t x2 .