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.

seq.1

@@ -31,7 +31,8 @@ as the output line format according to
 .It Fl s Ar sep
 Print
 .Ar sep
-between output lines. The default is "\en".
+between output lines.
+The default is "\en".
 .It Fl w
 Print out lines in equal width.
 .El