WordPress FTP file checking was previously based upon `ftp_nlist()`. This function can be problematic at scale with a directory containing a large number of files. The same issue occurred using it with ftpsockets.
This changeset rewrites the FTP `exists()` functions to utilize a more efficient and stable check.
Props giox069, desrosj, mkox, afragen, costdev, pbiron, peterwilsoncc.
Fixes#51170.
See #53318, #39781.
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git-svn-id: http://core.svn.wordpress.org/trunk@53419 1a063a9b-81f0-0310-95a4-ce76da25c4cd