Change rpc_set_context_args to nfs_set_context_args and make it static

It only makes sense to pass url arguments to an nfs URL and thus a nfs context.
While some arguments may apply to the underlying raw rpc context (uid, gid,
tcp settings etc) some future arguments may need to apply to the nfs context
instead, such as selecting nfs version, whether or not to force sync writes
etc.
Change rpc_mount_* to rpc_mount3_* and make it explicit which version of
the protocol we are using.
Fix some compiler warnings and bugs

Add missing %s to two example programs and fix incorrect argument to
rpc_nfs3_fsinfo_async()