Allow automatic renicing.

Issue: freebsd#178

src/etc/poudriere.conf.sample

@@ -230,6 +230,11 @@ DISTFILES_CACHE=/usr/ports/distfiles
 # which holdup the rest of the queue to build more quickly.
 #ALLOW_MAKE_JOBS_PACKAGES="pkg ccache py*"
 
+# By default poudriere does not nice(3) itself.  Set NICE to "yes"
+# to increment scheduler priority by 10, or any other value to use that.
+# Only root can set negative values.
+#NICE=no
+
 # Timestamp every line of build logs
 # Default: no
 #TIMESTAMP_LOGS=no

src/share/poudriere/common.sh

@@ -8625,6 +8625,17 @@ esac
 
 NCPU=$(sysctl -n hw.ncpu)
 
+if [ "${NICE:=no}" = "no" ]; then
+    :
+else
+    msg "renicing to NICE value '${NICE}'"
+    if [ "${NICE}" = "yes" ]; then
+        renice -n 10 -p $$
+    else
+        renice -n ${NICE} -p $$
+    fi
+fi
+
 # Determine if umount -n can be used.
 if grep -q "#define[[:space:]]MNT_NONBUSY" /usr/include/sys/mount.h \
     2>/dev/null; then