select.2, select_tut.2: Consolidate info on usleep() emulation in one place

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/select.2

@@ -287,14 +287,6 @@ struct timespec {
 .EE
 .in
 .PP
-Some code calls
-.BR select ()
-with all three sets empty,
-.I nfds
-zero, and a non-NULL
-.I timeout
-as a fairly portable way to sleep with subsecond precision.
-.PP
 On Linux,
 .BR select ()
 modifies
@@ -448,6 +440,18 @@ when
 .B _GNU_SOURCE
 is defined.
 .\"
+.SS Emulating usleep(3)
+.PP
+Before the advent of
+.BR usleep (3),
+some code employed a call to
+.BR select ()
+with all three sets empty,
+.I nfds
+zero, and a non-NULL
+.I timeout
+as a fairly portable way to sleep with subsecond precision.
+.\"
 .SS Correspondence between select() and poll() notifications
 Within the Linux kernel source,
 .\" fs/select.c

man2/select_tut.2

@@ -463,24 +463,6 @@ then the sets must be reinitialized before each call.
 .\" Having no file descriptors set is a useful
 .\" way to sleep the process with subsecond precision by using the timeout.
 .\" (See further on.)
-.SS Usleep emulation
-On systems that do not have a
-.BR usleep (3)
-function, you can call
-.BR select ()
-with a finite timeout and no file descriptors as
-follows:
-.PP
-.in +4n
-.EX
-struct timeval tv;
-tv.tv_sec = 0;
-tv.tv_usec = 200000;  /* 0.2 seconds */
-select(0, NULL, NULL, NULL, &tv);
-.EE
-.in
-.PP
-This is guaranteed to work only on UNIX systems, however.
 .SH RETURN VALUE
 On success,
 .BR select ()