Make xbps_fetch_file accept a callback to update its progress.

This also makes xbps_unpack_binary_pkg follow this convention by avoiding
static variables.

--HG--
branch : progress_callback
rename : bin/xbps-repo/util.c => bin/xbps-bin/util.c

bin/xbps-repo/Makefile

@@ -2,7 +2,8 @@ TOPDIR = ../..
 -include $(TOPDIR)/config.mk
 
 BIN =	xbps-repo
-OBJS =	main.o util.o index.o repository.o find-files.o
+OBJS =	main.o index.o repository.o find-files.o
+OBJS += ../xbps-bin/fetch.o ../xbps-bin/util.o
 MAN = $(BIN).8
 
 include $(TOPDIR)/prog.mk