Elimate use of the non-standard type systime_t and replace it the equivalent, standard type clock_t

Squashed commit of the following:

    sched:  Rename all use of system_t to clock_t.
    syscall:  Rename all use of system_t to clock_t.
    net:  Rename all use of system_t to clock_t.
    libs:  Rename all use of system_t to clock_t.
    fs:  Rename all use of system_t to clock_t.
    drivers:  Rename all use of system_t to clock_t.
    arch:  Rename all use of system_t to clock_t.
    include:  Remove definition of systime_t; rename all use of system_t to clock_t.

fs/procfs/fs_procfsuptime.c

@@ -208,7 +208,7 @@ static ssize_t uptime_read(FAR struct file *filep, FAR char *buffer,
   size_t linesize;
   off_t offset;
   ssize_t ret;
-  systime_t ticktime;
+  clock_t ticktime;
 
 #if defined(CONFIG_HAVE_DOUBLE) && defined(CONFIG_LIBC_FLOATINGPOINT)
   double now;

fs/vfs/fs_poll.c

@@ -408,7 +408,7 @@ int poll(FAR struct pollfd *fds, nfds_t nfds, int timeout)
         }
       else if (timeout > 0)
         {
-          systime_t ticks;
+          clock_t ticks;
 
           /* "Implementations may place limitations on the granularity of
            * timeout intervals. If the requested timeout interval requires