Squashed commit of the following:

This commit backs out most of commit b4747286b1.  That change was added because sem_wait() would sometimes cause cancellation points inappropriated.  But with these recent changes, nxsem_wait() is used instead and it is not a cancellation point.

    In the OS, all calls to sem_wait() changed to nxsem_wait().  nxsem_wait() does not return errors via errno so each place where nxsem_wait() is now called must not examine the errno variable.

    In all OS functions (not libraries), change sem_wait() to nxsem_wait().  This will prevent the OS from creating bogus cancellation points and from modifying the per-task errno variable.

    sched/semaphore:  Add the function nxsem_wait().  This is a new internal OS interface.  It is functionally equivalent to sem_wait() except that (1) it is not a cancellation point, and (2) it does not set the per-thread errno value on return.

fs/mmap/fs_munmap.c

@@ -1,7 +1,7 @@
 /****************************************************************************
  * fs/mmap/fs_munmap.c
  *
- *   Copyright (C) 2011 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2011, 2017 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -120,10 +120,11 @@ int munmap(FAR void *start, size_t length)
   /* Find a region containing this start and length in the list of regions */
 
   rammap_initialize();
-  ret = sem_wait(&g_rammaps.exclsem);
+  ret = nxsem_wait(&g_rammaps.exclsem);
   if (ret < 0)
     {
-      return ERROR;
+      errcode = ret;
+      goto errout;
     }
 
   /* Seach the list of regions */
@@ -205,6 +206,8 @@ int munmap(FAR void *start, size_t length)
 
 errout_with_semaphore:
   nxsem_post(&g_rammaps.exclsem);
+
+errout:
   set_errno(errcode);
   return ERROR;
 }

fs/mmap/fs_rammap.c

@@ -1,7 +1,7 @@
 /****************************************************************************
  * fs/mmap/fs_rammmap.c
  *
- *   Copyright (C) 2011 Gregory Nutt. All rights reserved.
+ *   Copyright (C) 2011, 2017 Gregory Nutt. All rights reserved.
  *   Author: Gregory Nutt <gnutt@nuttx.org>
  *
  * Redistribution and use in source and binary forms, with or without
@@ -220,10 +220,11 @@ FAR void *rammap(int fd, size_t length, off_t offset)
   /* Add the buffer to the list of regions */
 
   rammap_initialize();
-  ret = sem_wait(&g_rammaps.exclsem);
+  ret = nxsem_wait(&g_rammaps.exclsem);
   if (ret < 0)
     {
-      goto errout_with_errno;
+      errcode = -ret;
+      goto errout_with_region;
     }
 
   map->flink  = g_rammaps.head;
@@ -234,13 +235,16 @@ FAR void *rammap(int fd, size_t length, off_t offset)
 
 errout_with_region:
   kumm_free(alloc);
+
 errout:
   set_errno(errcode);
   return MAP_FAILED;
 
+#ifndef CONFIG_DEBUG_FS
 errout_with_errno:
   kumm_free(alloc);
   return MAP_FAILED;
+#endif
 }
 
 #endif /* CONFIG_FS_RAMMAP */