MSVC: Enable building ocspcheck.

- Add `STDIN_FILENO` to compat unistd header.
- Use quotes to include compat getopt header in the compat unistd.
- Export additional symbols needed by ocspcheck (optarg, optind,
  ftruncate)

apps/ocspcheck/CMakeLists.txt

@@ -1,5 +1,3 @@
-if(NOT MSVC)
-
 set(
 	OCSPCHECK_SRC
 	http.c
@@ -33,5 +31,3 @@ if(ENABLE_LIBRESSL_INSTALL)
 	install(FILES ocspcheck.8 DESTINATION ${CMAKE_INSTALL_MANDIR}/man8)
 
 endif(ENABLE_LIBRESSL_INSTALL)
-
-endif()