tree-wide: use 'signed int' instead of 'int' for bit field variables

Suggested by LGTM: https://lgtm.com/rules/1506024027114/

src/core/unit.h

@@ -352,7 +352,7 @@ typedef struct Unit {
 
         /* When writing transient unit files, stores which section we stored last. If < 0, we didn't write any yet. If
          * == 0 we are in the [Unit] section, if > 0 we are in the unit type-specific section. */
-        int last_section_private:2;
+        signed int last_section_private:2;
 } Unit;
 
 typedef struct UnitStatusMessageFormats {

src/libsystemd/sd-event/sd-event.c

@@ -92,7 +92,7 @@ struct sd_event_source {
         char *description;
 
         EventSourceType type:5;
-        int enabled:3;
+        signed int enabled:3;
         bool pending:1;
         bool dispatching:1;
         bool floating:1;