Fix typos in manpages
windows build: do not export externals from static library

If you are building a DLL, then you need to explicitly export each
entry point. When building a static library, you should not.

libssh2 was exporting the entry points whether it was building a DLL or a
static library. To elaborate further, if libssh2 was used as a static
library, which was being linked into a DLL, the libssh2 API would be
exported from that separate DLL.