Fix GH-10521: ftp_get/ftp_nb_get resumepos offset is maximum 10GB

The char arrays were too small for a long on 64-bit systems, which
resulted in cutting off the string at the end with a

Fix GH-10521: ftp_get/ftp_nb_get resumepos offset is maximum 10GB

The char arrays were too small for a long on 64-bit systems, which
resulted in cutting off the string at the end with a NUL byte. Use a
size of MAX_LENGTH_OF_LONG to fix this issue instead of a fixed size
of 11 chars.

Closes GH-10525.

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