[FIX] ir_attachment: fix security issues on ir_attachment

[FIX] ir_attachment: fix security issues on ir_attachment
check: verify the permissions even when no ids are passed (skipped permission checking for create)
create: verify has the write access on the related model (instead of create, was not checked anyway)
function field: execute the write in fnct_inv as superuser (was impossible to have creation without write access)

bzr revid: mat@openerp.com-20131029171420-x87wu7ph8ej7mtro