Move sys/select.h functions into their own directory.

libc/Makefile

@@ -237,7 +237,6 @@ remove.o \
 renameat.o \
 rename.o \
 scanf.o \
-select.o \
 signal/kill.o \
 signal/psignal.o \
 signal/raise.o \
@@ -259,6 +258,7 @@ stdlib/rand.o \
 stdlib/realpath.o \
 stdlib/system.o \
 sys/display/dispmsg_issue.o \
+sys/select/select.o \
 sys/socket/accept4.o \
 sys/socket/accept.o \
 sys/socket/bind.o \

libc/sys/select/select.cpp

@@ -17,7 +17,7 @@
     You should have received a copy of the GNU Lesser General Public License
     along with the Sortix C Library. If not, see <http://www.gnu.org/licenses/>.
 
-    select.cpp
+    sys/select/select.cpp
     Waiting on multiple file descriptors.
 
 *******************************************************************************/