xbps-src: improve run_func and always make a log in $wrksrc.

Also when or after the pkg is installed to destdir/stowned, and there was
any error, always remove the files that were installed into destdir.

With these changes there's no need to return any value in the do_foo()
functions because they are caught automatically. Also any function is now
logged into its own file to always log what happens.

xbps-src/shutils/fetch_funcs.sh

@@ -70,7 +70,7 @@ fetch_distfiles()
 
 	if [ -n "$nofetch" ]; then
 		msg_normal "Package '$pkgname ($lver)': running do_fetch phase."
-		cd ${XBPS_BUILDDIR} && run_func do_fetch 2>/dev/null && return $?
+		cd ${XBPS_BUILDDIR} && run_func do_fetch && return $?
 	fi
 
 	cd $XBPS_SRCDISTDIR || return 1