!sys/types.h: change time_t and clock_t to int64_t to align with other OSes

POSIX leaves the signedness of time_t and clock_t unspecified, but
mainstream implementations (Linux glibc/musl, the BSDs, macOS, RTEMS,
Zephyr's POSIX layer, Windows _time64) expose time_t as signed 64-bit.
NuttX has historically used uint64_t only because it was tied to the
CONFIG_SYSTEM_TIME64 knob; with that gone, switch:

  time_t   : uint64_t  -> int64_t
  clock_t  : uint64_t  -> int64_t
  CLOCK_MAX: UINT64_MAX -> INT64_MAX

This lets (time_t)-1 sentinels, negative tick deltas, and host-side
headers behave as on every other POSIX system without source churn.

Headers updated:
  - include/sys/types.h, include/limits.h, include/nuttx/clock.h
  - include/nuttx/fs/hostfs.h (nuttx_time_t alias)
  - include/nuttx/{mqueue.h,wdog.h,wqueue.h,timers/clkcnt.h}

Because clock_t is now signed 64-bit, the NuttX-internal sclock_t
alias becomes redundant: every sclock_t/SCLOCK_MAX use is folded
back to clock_t/CLOCK_MAX (notably in sched/wdog, sched/mqueue,
sched/sched, sched/clock, sched/timer, libs/libc/time, fs/vfs and
the drivers/arch consumers below).

Tick/period constants (NSEC_PER_SEC, USEC_PER_SEC, MSEC_PER_SEC,
SEC_PER_MIN, ...) in include/nuttx/clock.h are retyped from "long"
literals to INT64_C(...) so that 64-bit arithmetic no longer
depends on the host's long width.

Strip now-redundant (time_t)/(clock_t)/(unsigned long) casts and
unsigned-only branches across the tree:
  - arch RTC / oneshot / tickless lowerhalfs:
      arm: cxd56xx, efm32, imxrt, lc823450, max326xx, sam34, sama5,
           samd5e5, samv7, stm32, stm32f7, stm32h7, stm32l4, stm32wb,
           xmc4
      mips: pic32mz       sparc: bm3803       x86_64: intel64
      risc-v/xtensa: espressif (esp_i2c[_slave], esp_rtc,
           esp32c3{_i2c,_rtc,_wifi_adapter}, esp32{,s2,s3}_*),
           mpfs_perf
  - drivers: audio/tone, input/aw86225, power/pm/{activity,
           stability}_governor, rpmsg/rpmsg_ping,
           timers/{ds3231,mcp794xx,pcf85263,rx8010},
           wireless/ieee80211/bcm43xxx, wireless/spirit/spirit_spi
  - core: fs/vfs/{fs_poll,fs_timerfd}, mm/iob/iob_alloc,
          libs/libc/{netdb/lib_dnscache,time/{lib_calendar2utc,
          lib_time}}, net/icmp/icmp_pmtu, net/icmpv6/icmpv6_pmtu,
          net/ipfrag, net/tcp/{tcp.h,tcp_timer},
          net/utils/net_snoop, net/mld/mld_query (drop the now-dead
          mld_mrc2mrd helper since signed math handles it directly),
          sched/clock/{clock,clock_initialize},
          sched/sched/{sched_profil,sched_setparam,sched_setscheduler},
          sched/pthread/pthread_create,
          sched/wdog/{wd_gettime,wd_start,wdog.h},
          sched/timer/timer_gettime, sched/mqueue/*

Flip the few in-tree printf format strings that assumed an
unsigned 64-bit tv_sec:
  * drivers/rpmsg/rpmsg_ping.c                       PRIu64 -> PRId64
  * arch/xtensa/src/esp32{,s2,s3}/esp32*_oneshot_lowerhalf.c
                                          PRIu32 (already wrong) -> PRId64

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/netdb/lib_dnscache.c

@@ -145,7 +145,7 @@ void dns_save_answer(FAR const char *hostname,
   /* Get the current time */
 
   clock_gettime(CLOCK_MONOTONIC, &now);
-  entry->ctime = (time_t)now.tv_sec;
+  entry->ctime = now.tv_sec;
 #endif
 
   strlcpy(entry->name, hostname, CONFIG_NETDB_DNSCLIENT_NAMESIZE);
@@ -212,7 +212,7 @@ int dns_find_answer(FAR const char *hostname, FAR union dns_addr_u *addr,
   FAR struct dns_cache_s *entry;
 #if CONFIG_NETDB_DNSCLIENT_LIFESEC > 0
   struct timespec now;
-  uint32_t elapsed;
+  time_t elapsed;
   int ret;
 #endif
   int next;
@@ -243,12 +243,9 @@ int dns_find_answer(FAR const char *hostname, FAR union dns_addr_u *addr,
         }
 
 #if CONFIG_NETDB_DNSCLIENT_LIFESEC > 0
-      /* Check if this entry has expired
-       * REVISIT: Does not this calculation assume that the sizeof(time_t)
-       * is equal to the sizeof(uint32_t)?
-       */
+      /* Check if this entry has expired */
 
-      elapsed = (uint32_t)now.tv_sec - (uint32_t)entry->ctime;
+      elapsed = now.tv_sec - entry->ctime;
       if (ret >= 0 &&
           (elapsed > CONFIG_NETDB_DNSCLIENT_LIFESEC || elapsed > entry->ttl))
         {