Change sizes in the API to use a libssh2_size_t

[eschaton]

Because it's already widely deployed, libssh2 can't just switch its APIs to use size_t and ssize_t without breaking binary compatibility. However, it could define a couple of new types (like libssh2_size_t and libssh2_ssize_t) and use them instead. Whether these are defined in a binary-compatible way or defined as covers for size_t and ssize_t could be a confiruration option.

The latter would be useful to those embedding libssh2 who don't have to worry about binary compatibility, it would also allow building it with the system size_t locally just to run tests, and finally it would allow new platforms that don't yet have binary compatibility restrictions to use the proper types.

[stale]

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