tidy-up: gettimeofday() fallback and use

Simplify the way we handle `gettimeofday()` fallback for platforms
without native support or without any support. Make it similar to
how we handle `snprintf()`.

In case of no native `gettimeofday()` support and a non-Windows
platform, our local fallback returns zero in `tv_usec` and `tv_sec`,
ending up with a zero `timeout_remaining` in `session.c`, same as
before this patch.

Also:
- drop unused `sys/time.h` headers.
- fix our fallback code to compile with any Windows compilers
  (not just MSVC)
- delete unnecessary casts.

Closes #1001

example/ssh2_agent_forwarding.c

@@ -29,9 +29,6 @@
 #ifdef HAVE_ARPA_INET_H
 #include <arpa/inet.h>
 #endif
-#ifdef HAVE_SYS_TIME_H
-#include <sys/time.h>
-#endif
 
 #include <sys/types.h>
 #include <fcntl.h>