sys_netbsd: allow running without root privileges

On NetBSD programs with write access to /dev/clockctl can adjust or set
the system clock without the root privileges. Add a function to drop the
privileges and check if the process has write access to the device to
get a more descriptive error message when the chrony uid/gid doesn't
match the owner of the device.

configure

@@ -215,6 +215,7 @@ feat_rtc=1
 try_rtc=0
 feat_droproot=1
 try_libcap=0
+try_clockctl=0
 readline_lib=""
 readline_inc=""
 ncurses_lib=""
@@ -405,6 +406,7 @@ case $SYSTEM in
     NetBSD-* )
         EXTRA_OBJECTS="sys_netbsd.o"
         EXTRA_LIBS="-lkvm"
+        try_clockctl=1
         add_def NETBSD
         echo "Configuring for $SYSTEM"
     ;;
@@ -590,6 +592,12 @@ then
     EXTRA_LIBS="$EXTRA_LIBS -lcap"
 fi
 
+if [ $feat_droproot = "1" ] && [ $try_clockctl = "1" ] && \
+  test_code '<sys/clockctl.h>' 'sys/clockctl.h' '' '' ''
+then
+  add_def FEAT_PRIVDROP
+fi
+
 if [ $feat_rtc = "1" ] && [ $try_rtc = "1" ] && \
   test_code '<linux/rtc.h>' 'sys/ioctl.h linux/rtc.h' '' '' \
     'ioctl(1, RTC_UIE_ON&RTC_UIE_OFF&RTC_RD_TIME&RTC_SET_TIME, 0&RTC_UF);'