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.

sort.1

@@ -26,8 +26,9 @@ reads from stdin.
 .It Fl C
 Check that the concatenation of the given
 .Ar files
-is sorted rather than sorting them. In this mode, no output is printed to
-stdout, and the exit status indicates the result of the check.
+is sorted rather than sorting them.
+In this mode, no output is printed to stdout, and the exit status
+indicates the result of the check.
 .It Fl b
 Skip leading whitespace of columns when sorting.
 .It Fl c
@@ -62,7 +63,8 @@ where
 and
 .Sy e
 are the starting column, starting character in that column, ending column and
-the ending character of that column respectively. If they are not specified,
+the ending character of that column respectively.
+If they are not specified,
 .Sy s
 refers to the first character of the specified starting column,
 .Sy E