Add system filesize resource limit to wp_max_upload_size function

[yani.iliev]

A file will that is smaller than wp_max_upload_size but larger than the soft filesize resource limit will not upload successfully.
​posix_getrlimit will return system resource limits.
wp_max_upload_size function should return the minimum of upload_max_filesize, post_max_size, and posix_getrlimit()[soft filesize].