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.

net/tcp/tcp.h

@@ -205,7 +205,7 @@ struct tcp_conn_s
    * system clock tick.
    */
 
-  systime_t  keeptime;    /* Last time that the TCP socket was known to be
+  clock_t    keeptime;    /* Last time that the TCP socket was known to be
                            * alive (ACK or data received) OR time that the
                            * last probe was sent. */
   uint16_t   keepidle;    /* Elapsed idle time before first probe sent (dsec) */

net/tcp/tcp_send_unbuffered.c

@@ -107,7 +107,7 @@ struct send_s
   uint32_t                snd_isn;     /* Initial sequence number */
   uint32_t                snd_acked;   /* The number of bytes acked */
 #ifdef CONFIG_NET_SOCKOPTS
-  systime_t               snd_time;    /* Last send time for determining timeout */
+  clock_t                 snd_time;    /* Last send time for determining timeout */
 #endif
 #if defined(CONFIG_NET_TCP_SPLIT)
   bool                    snd_odd;     /* True: Odd packet in pair transaction */

net/tcp/tcp_sendfile.c

@@ -106,7 +106,7 @@ struct sendfile_s
   uint32_t           snd_isn;     /* Initial sequence number */
   uint32_t           snd_acked;   /* The number of bytes acked */
 #ifdef CONFIG_NET_SOCKOPTS
-  systime_t          snd_time;    /* Last send time for determining timeout */
+  clock_t            snd_time;    /* Last send time for determining timeout */
 #endif
 };