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.

pwd.1

@@ -13,7 +13,8 @@ prints the path of the current working directory.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl L
-Logical path, uses $PWD. This is the default.
+Logical path, uses $PWD.
+This is the default.
 .It Fl P
 Physical path, avoids all symlinks.
 .El