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.

strings.1

@@ -25,11 +25,13 @@ reads from stdin.
 .It Fl a
 Scan each
 .Ar file
-entirely. This is the default.
+entirely.
+This is the default.
 .It Fl n Ar num
 Print sequences of at least
 .Ar num
-characters.  The default is 4.
+characters.
+The default is 4.
 .It Fl t Ar format
 Prepend each string with its byte offset, with
 .Ar format