wrong usage of t() and form_plural()

While extracting pot files from ecommerce modules, i've found some wrong usage of t() and form_plural() in cart.module, ecommailer.module, and product.module.

basically, there was t() functions calls instead of strings used in form_plural(), which is not correct as form_plural calls in turn t(). (http://drupaldocs.org/api/head/function/format_plural). and there was also php variables concatenated to strings in t(), instead of supplying them as parameter to the function, making a single translated string for each variable values.

patch is attached to correct the function calls.

additionnally, i also had errors in product.module, but they need to be discussed before fixing :)

some t() functions are called with an object value instead of strings, line 995 (there's also the same thing for a form_plural:

t($p->notification->subject, $variables), t($p->notification->body, $variables)

the $p->notification->subject/body should already be translated, if i'm not mistaken, as they are entered by the shop admin. so we could simply remove the t() call, but $variables is an array containing all values for subsitition (%site, %renewal_link, ...). i think the t() function call also handle the feature of replacing values in notification emails, so we cannot remove the t() calls without removing this, still if i'm not mistaken of course. but in that case, i don't really like the idea to use t() for this. but i'll look at it further.

the last issue if with a plural form in product.module at line 877, but i'll product a patch for solving it, as it leads to a lot of translation of already translated items, including passing a translated string to strtotime, so this has to be studied a bit more :)

thanks