Work around broken HHVM ini_get() for 'upload_max_filesize' and 'post_max_size'

Work around broken HHVM ini_get() for 'upload_max_filesize' and 'post_max_size'

In HHVM, the settings 'upload_max_filesize' and 'post_max_size' are
not available via ini_get() due to some long-standing bug
(https://github.com/facebook/hhvm/issues/4993). Instead, one can use
'hhvm.server.upload.upload_max_file_size' and 'hhvm.server.max_post_size'
(in a typical PHP fashion, their names are subtly different than the
originals as to increase the potential for confusion).

Added a new method UploadBase::getMaxPhpUploadSize() to handle this.

Additionally:
* 'post_max_size' can be set to 0, which is equivalent to no limit.
  Handle this correctly.
* $wgMaxUploadSize can be an array structure, instead of just a number.
  Handle this correctly by using UploadBase::getMaxUploadSize().
* When no maximum is set, use PHP_INT_MAX rather than 1e100. It should
  be big enough, and the latter is a float, results in 0 when cast to
  int, and doesn't look as pretty when formatted in GB in the interface.

Bug: T116347
Change-Id: Idf707253eeae1b90792a7e26d2ab66d1317e67ae