* Move `_purple_fstat` to silc and rename to `silcpurple_fstat`
* Replace unknown `g_fchmod` with `fchmod`
* Fix `PurpleProtocol*` prototypes
* Derive `SilcPurpleXfer` from `PurpleXfer`
Testing Done:
Compile.
Bugs closed: PIDGIN-17436
Reviewed at https://reviews.imfreedom.org/r/481/
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