Multiple changes to pkgs on hold mode.

- Added transaction stats for pkgs on hold.

- Always add packages on hold to the transaction dictionary,
  its type will be set to XBPS_TRANS_HOLD.

- Changed xbps_transaction_update_pkg() to have a new "force"
  bool argument to force an update with a pkg on hold.

- As discussed in #274 with @Duncaen the only way to update a
  pkg on hold is by using `-f`, i.e `xbps-install -f foo`.

Closes #265
Closes #274

tests/xbps/libxbps/shell/hold_test.sh

@@ -120,9 +120,25 @@ keep_on_update_body() {
 	xbps-rindex -d -a $PWD/*.xbps
 	atf_check_equal $? 0
 	cd ..
+	# no update
+	xbps-install -r root --repository=$PWD/repo -yuvd
+	atf_check_equal $? 0
+	out=$(xbps-query -r root -p pkgver A)
+	atf_check_equal $out A-1.0_1
+	# no update without -f
 	xbps-install -r root --repository=$PWD/repo -yuvd A
 	atf_check_equal $? 0
 	out=$(xbps-query -r root -p pkgver A)
+	atf_check_equal $out A-1.0_1
+	# no update with -fu
+	xbps-install -r root --repository=$PWD/repo -yuvdf
+	atf_check_equal $? 0
+	out=$(xbps-query -r root -p pkgver A)
+	atf_check_equal $out A-1.0_1
+	# update with -f
+	xbps-install -r root --repository=$PWD/repo -yuvdf A
+	atf_check_equal $? 0
+	out=$(xbps-query -r root -p pkgver A)
 	atf_check_equal $out A-1.1_1
 	out=$(xbps-query -r root -p hold A)
 	atf_check_equal $out yes