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.

flock.1

@@ -11,15 +11,17 @@
 .Ar cmd Op arg ...
 .Sh DESCRIPTION
 .Nm
-is used to manage advisory locks on open files.  It is commonly used to prevent
-long running cron jobs from running in parallel.  If
+is used to manage advisory locks on open files.
+It is commonly used to prevent long running cron jobs from running in
+parallel.
+If
 .Ar file
 does not exist, it will be created.
 .Sh OPTIONS
 .Bl -tag -width Ds
 .It Fl n
-Set non-blocking mode on the lock.  Fail immediately if the lock
-cannot be acquired.
+Set non-blocking mode on the lock.
+Fail immediately if the lock cannot be acquired.
 .It Fl o
 Close the file descriptor before exec to avoid having the exec'ed
 program holding on to the lock.
@@ -28,5 +30,6 @@ Acquire a shared lock.
 .It Fl u
 Release the lock.
 .It Fl x
-Acquire an exclusive lock.  This is the default.
+Acquire an exclusive lock.
+This is the default.
 .El