ftp/libfilezilla: update 0.13.0 -> 0.15.1, taking maintainer

• Add argument to fz::strtok to return empty tokens
• Fix compatibility issue with Nettle < 3.3
• Fix fz::random_bytes on MinGW
• Fix memory leak in fz::buffer
• now depends on Nettle >= 3.1
• Added fz::sha512, fz::sha256, fz::sha1 and fz::md5 hash functions
• Added fz::hash_accumulator
• Added fz::hmac_sha256 HMAC function
• Added asymmetric encryption scheme using X25519
• Added signature scheme using Ed25519
• Changed and documented semantics of the return value of fz::remove_file, removing a non-existing file is not an error
• Added fz::equal_insensitive_ascii
• Added insensitive_ascii parameter to fz::starts_with and fz::ends_with
• Fixed namespace of to_wstring in wx glue