[stdlib] Changes to support Foundation on OpenBSD.

* Not implementing POSIXError for a given platform is not a blocking
  problem to getting a successful build of Swift. However, it is
  part of the validation tests (albeit locked behind REQUIRES) and is
  referenced when building Foundation. Implement by forklifting from
  `sys/errno.h`.

* Some Foundation class implementations require some missing includes in
  the platform modulemap. Add these.

stdlib/public/Platform/libc-openbsd.modulemap.gyb

@@ -72,10 +72,18 @@ module SwiftGlibc [system] {
       header "${GLIBC_INCLUDE_PATH}/string.h"
       export *
     }
+    module sysexits {
+      header "${GLIBC_INCLUDE_PATH}/sysexits.h"
+      export *
+    }
     module time {
       header "${GLIBC_INCLUDE_PATH}/time.h"
       export *
     }
+    module util {
+      header "${GLIBC_INCLUDE_PATH}/util.h"
+      export *
+    }
   }
 
   // POSIX
@@ -241,6 +249,10 @@ module SwiftGlibc [system] {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/mman.h"
         export *
       }
+      module mount {
+        header "${GLIBC_ARCH_INCLUDE_PATH}/sys/mount.h"
+        export *
+      }
       module msg {
         header "${GLIBC_ARCH_INCLUDE_PATH}/sys/msg.h"
         export *