Filter max file size in import uploader

[donncha]

It would be really useful if the max file size in wp_import_upload_form() could be modified by filters.

[rob1n]

(In [5527]) Filter max upload file size for import form. Props Donncha. fixes #4320

[markjaquith]

Keep in mind that this only allows you to reduce the max_upload_size... not increase it beyond the limit specified in php.ini

In order to support increasing, we'd have to:

1. compare ini_get() to the new value, to see if the new value is bigger
2. try to ini_set()
3. use ini_get() to see if it worked
4. use ini_set() when handling the upload