xbps_file_exec: simplify and don't chroot when rootdir == /.

Before that change we were chroot(2)ing even when rootdir was set to /.

include/xbps_api_impl.h

@@ -60,7 +60,7 @@ __BEGIN_DECLS
 
 /**
  * @private
- * From lib/dewey.c
+ * From lib/external/dewey.c
  */
 int HIDDEN dewey_match(const char *, const char *);
 
@@ -165,11 +165,9 @@ char HIDDEN *xbps_get_remote_repo_string(const char *);
 
 /**
  * @private
- * From lib/fexec.c
+ * From lib/external/fexec.c
  */
 int HIDDEN xbps_file_exec(const char *, ...);
-int HIDDEN xbps_file_exec_skipempty(const char *, ...);
-int HIDDEN xbps_file_chdir_exec(const char *, const char *, ...);
 
 /**
  * @private