core: set private section name for automount units

Because this was left unset, the unit_write_setting() function was
refusing to write out the automount-specific TimeoutIdleSec= and
DirectoryMode= settings when creating transient automount units.
Set it to the proper value in line with other unit types.

hwdb.d/60-keyboard.hwdb

@@ -526,10 +526,15 @@ evdev:atkbd:dmi:bvn*:bvr*:bd*:svnHewlett-Packard*:pnHP*Pavilion*dv7*Notebook*PC:
  KEYBOARD_KEY_c6=break
  KEYBOARD_KEY_94=reserved
 
-# Pavilion and Spectre x360 13 (Prevents random airplane mode activation)
+# Pavilion 13 x360 (Tablet mode and SYSRQ key)
+evdev:atkbd:dmi:bvn*:bvr*:bd*:svnHewlett-Packard*:pn*[pP][aA][vV][iI][lL][iI][oO][nN]*13*x360*:pvr*
+ KEYBOARD_KEY_d7=!f22                                   # touchpad off
+ KEYBOARD_KEY_d9=unknown
+ KEYBOARD_KEY_d2=sysrq                                  # Fn+Print = SYSRQ
+
+# Spectre x360 13 (Prevents random airplane mode activation)
 evdev:atkbd:dmi:bvn*:bvr*:bd*:svnHewlett-Packard*:pn*[sS][pP][eE][cC][tT][rR][eE]*x360*13*:pvr*
 evdev:atkbd:dmi:bvn*:bvr*:bd*:svnHP*:pn*[sS][pP][eE][cC][tT][rR][eE]*x360Convertible*:pvr*
-evdev:atkbd:dmi:bvn*:bvr*:bd*:svnHewlett-Packard*:pn*[pP][aA][vV][iI][lL][iI][oO][nN]*13*x360*:pvr*
  KEYBOARD_KEY_d7=unknown
 
 # Spectre x360 13

src/core/automount.c

@@ -1085,6 +1085,7 @@ const UnitVTable automount_vtable = {
                 "Unit\0"
                 "Automount\0"
                 "Install\0",
+        .private_section = "Automount",
 
         .can_transient = true,
         .can_fail = true,