xbps-install(1): added -I, --ignore-file-conflicts.

If set it will continue with the transaction and will
just print what are the conflicting files without returning
EEXIST.

This is a temporary solution for void where there are still
some packages with conflicting files (qt5-host-tools vs qt5-tools-devel).

With input by @duncaen

tests/xbps/libxbps/shell/obsoletefiles_test.sh

@@ -563,6 +563,10 @@ conflicting_files_in_transaction_body() {
 	xbps-install -r root -C null.conf --repository=$PWD/some_repo -ydv A B
 	# EEXIST
 	atf_check_equal $? 17
+
+	# ignored file conflicts
+	xbps-install -r root -C null.conf --repository=$PWD/some_repo -Iydv A B
+	atf_check_equal $? 0
 }
 
 atf_test_case obsolete_dir