ntpquery: Add a '-a' flag that makes it use adjtime

With this, `ntpquery` can adjust the system time without
making it jump.

A fun activity with this in:

0. Boot
1. Run `su`
2. Run `ntpquery -a` to adjust the time offset after boot
   (usually around a second)
3. Keep running `ntpquery ; adjtime` to see how the offset
   behind NTP and the remaining adjtime both shrink.
   adjtime adjustment is large enough to make the time offset
   go down by a bit, but we currently lose time quickly enough
   that by the time adjtime is done, we've only corrected the
   clock about halfway, and not all the way to zero. Goto 2.

So this isn't all that great yet, but I think it's good enough
to think about turning this into a permanently running service next.

Userland/ntpquery.cpp

@@ -113,6 +113,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
+    bool adjust_time = false;
     bool set_time = false;
     bool verbose = false;
     // FIXME: Change to serenityos.pool.ntp.org once https://manage.ntppool.org/manage/vendor/zone?a=km5a8h&id=vz-14154g is approved.
@@ -127,12 +128,18 @@ int main(int argc, char** argv)
     // - time.google.com , https://developers.google.com/time/smear , linear-smears over 24 hours
     const char* host = "time.google.com";
     Core::ArgsParser args_parser;
-    args_parser.add_option(set_time, "Adjust system time (requires root)", "set", 's');
+    args_parser.add_option(adjust_time, "Gradually adjust system time (requires root)", "adjust", 'a');
+    args_parser.add_option(set_time, "Immediately set system time (requires root)", "set", 's');
     args_parser.add_option(verbose, "Verbose output", "verbose", 'v');
     args_parser.add_positional_argument(host, "NTP server", "host", Core::ArgsParser::Required::No);
     args_parser.parse(argc, argv);
 
-    if (!set_time) {
+    if (adjust_time && set_time) {
+        fprintf(stderr, "-a and -s are mutually exclusive\n");
+        return 1;
+    }
+
+    if (!adjust_time && !set_time) {
         if (pledge("stdio inet dns", nullptr) < 0) {
             perror("pledge");
             return 1;
@@ -145,7 +152,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    if (pledge(set_time ? "stdio inet settime" : "stdio inet", nullptr) < 0) {
+    if (pledge((adjust_time || set_time) ? "stdio inet settime" : "stdio inet", nullptr) < 0) {
         perror("pledge");
         return 1;
     }
@@ -311,4 +318,19 @@ int main(int argc, char** argv)
     if (verbose)
         printf("Delay: %f\n", delay_s);
     printf("Offset: %f\n", offset_s);
+
+    if (adjust_time) {
+        long delta_us = static_cast<long>(round(offset_s * 1'000'000));
+        timeval delta_timeval;
+        delta_timeval.tv_sec = delta_us / 1'000'000;
+        delta_timeval.tv_usec = delta_us % 1'000'000;
+        if (delta_timeval.tv_usec < 0) {
+            delta_timeval.tv_sec--;
+            delta_timeval.tv_usec += 1'000'000;
+        }
+        if (adjtime(&delta_timeval, nullptr) < 0) {
+            perror("adjtime set");
+            return 1;
+        }
+    }
 }