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.

tail.1

@@ -13,7 +13,8 @@
 .Nm
 writes the last 10 lines of each
 .Ar file
-to stdout. If no
+to stdout.
+If no
 .Ar file
 is given,
 .Nm
@@ -23,15 +24,16 @@ reads from stdin.
 .It Fl c Ar num | Fl m Ar num | Fl n Ar num | Fl Ns Ar num
 Display final
 .Ar num
-bytes | characters | lines |
-lines. If
+bytes | characters | lines | lines.
+If
 .Ar num
 begins with '+'
 it is an offset from the beginning of each
 .Ar file .
 If
 .Ar num
-begins with '-' it is as if no sign was given. The default is 10 lines.
+begins with '-' it is as if no sign was given.
+The default is 10 lines.
 .It Fl f
 If one
 .Ar file