Replacing timer_t by netbsd's timer_t

* Renamed struct timer to struct minix_timer
 * Renamed timer_t to minix_timer_t
 * Ensured all the code uses the minix_timer_t typedef
 * Removed ifdef around _BSD_TIMER_T
 * Removed include/timers.h and merged it into include/minix/timers.h
 * Resolved prototype conflict by renaming kernel's (re)set_timer
   to (re)set_kernel_timer.

Change-Id: I56f0f30dfed96e1a0575d92492294cf9a06468a5

servers/devman/buf.c

@@ -9,7 +9,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <lib.h>
-#include <timers.h>
+#include <minix/timers.h>
 
 #include <minix/callnr.h>
 #include <minix/type.h>

servers/devman/devman.h

@@ -10,7 +10,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <lib.h>
-#include <timers.h>
+#include <minix/timers.h>
 
 #include <minix/callnr.h>
 #include <minix/type.h>

servers/devman/main.c

@@ -7,7 +7,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <lib.h>
-#include <timers.h>
+#include <minix/timers.h>
 
 #include <minix/callnr.h>
 #include <minix/type.h>