bootctl: rework/modernize "unlink" and add Varlink API for it

Among other things this changes tracking of the location of resources
during GC from using the BootEntrySource enum rather than a path, since
we have that and it is more efficient and easier to grok.

src/bootctl/bootctl.h

@@ -51,6 +51,7 @@ extern char *arg_certificate_source;
 extern char *arg_private_key;
 extern KeySourceType arg_private_key_source_type;
 extern char *arg_private_key_source;
+extern bool arg_oldest;
 
 static inline const char* arg_dollar_boot_path(void) {
         /* $BOOT shall be the XBOOTLDR partition if it exists, and otherwise the ESP */