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.

cut.1

@@ -33,9 +33,10 @@ reads from stdin.
 .Pp
 .Ar list
 is a comma or space separated list of numbers and ranges starting
-from 1. Ranges have the form 'N-M'. If N or M is missing,
-beginning or end of line is assumed. Numbers and ranges
-may be repeated, overlapping and in any order.
+from 1.
+Ranges have the form 'N-M'. If N or M is missing, beginning or end
+of line is assumed.
+Numbers and ranges may be repeated, overlapping and in any order.
 .Pp
 Selected input is written in the same order it is read
 and is written exactly once.
@@ -47,16 +48,17 @@ specifies byte | character positions.
 .It Fl d Ar delim
 Use
 .Ar delim
-as field delimiter, which can be an arbitrary string. Default is '\et'.
+as field delimiter, which can be an arbitrary string.
+Default is '\et'.
 .It Fl f Ar list
 .Ar list
-specifies field numbers. Lines not containing field
-delimiters are passed through, unless
+specifies field numbers.
+Lines not containing field delimiters are passed through, unless
 .Fl s
 is specified.
 .It Fl n
-Do not split multibyte characters. A character is written when its
-last byte is selected.
+Do not split multibyte characters.
+A character is written when its last byte is selected.
 .It Fl s
 Suppress lines not containing field delimiters.
 .El