[forms] fix form_needs_multipart implementation
problem
-------
The current implementation of form_needs_multipart goes down
recursively in all inlined forms to check if one of them needs
a multipart enctype.
If we have a relation definition as X part_of X, the previous
implementation potentially lead to an infinite recursion
/var/log/cubicweb/*.log {
weekly
missingok
rotate 10
compress
delaycompress
notifempty
create 640 root adm
sharedscripts
postrotate
if [ -x /usr/sbin/invoke-rc.d ]; then \
invoke-rc.d cubicweb reload > /dev/null; \
else \
/etc/init.d/cubicweb reload > /dev/null; \
fi; \
endscript
}