Support Illumos/Solaris fine-grained privileges

Includes a pre-auth privsep sandbox and several pledge()
emulations. bz#2511, patch by Alex Wilson.

ok dtucker@
diff --git a/platform.h b/platform.h
index 1c7a45d..e687c99 100644
--- a/platform.h
+++ b/platform.h
@@ -31,3 +31,8 @@
 char *platform_get_krb5_client(const char *);
 char *platform_krb5_get_principal_name(const char *);
 int platform_sys_dir_uid(uid_t);
+
+/* in platform-pledge.c */
+void platform_pledge_agent(void);
+void platform_pledge_sftp_server(void);
+void platform_pledge_mux(void);