Fix SILC compilation

* 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/