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.

nl.1

@@ -40,7 +40,8 @@ All lines.
 .It n
 No lines.
 .It t
-Only non-empty lines. This is the default.
+Only non-empty lines.
+This is the default.
 .It p Ns Ar expr
 Only lines matching
 .Ar expr
@@ -50,11 +51,13 @@ according to
 .It Fl d Ar delim
 Set
 .Ar delim
-as the delimiter for logical pages. If
+as the delimiter for logical pages.
+If
 .Ar delim
 is only one character,
 .Nm
-appends ":" to it. The default is "\e:".
+appends ":" to it.
+The default is "\e:".
 .It Fl i Ar num
 Set the increment between numbered lines to
 .Ar num .
@@ -70,14 +73,16 @@ to one of:
 .It ln
 Left justified.
 .It rn
-Right justified. This is the default.
+Right justified.
+This is the default.
 .It rz
 Right justified with leading zeroes.
 .El
 .It Fl s Ar sep
 Use
 .Ar sep
-to separate line numbers and lines. The default is "\et".
+to separate line numbers and lines.
+The default is "\et".
 .It Fl v Ar num
 Start counting lines from
 .Ar num .