Add POSIX fcntl to control blocking

Add the POSIX fcntl call, but only implementing F_SETFL and F_GETFL
for O_NONBLOCK, so users can control the blocking flag of streams
with only the integer file descriptor.

Necessary to portably control the blockingness of the console:

    int flags = fcntl(STDOUT_FILENO, F_GETFL);
    fcntl(STDOUT_FILENO, F_SETFL, flags | O_NONBLOCK);

platform/mbed_retarget.cpp

@@ -862,6 +862,41 @@ extern "C" int fstat(int fildes, struct stat *st) {
     return 0;
 }
 
+extern "C" int fcntl(int fildes, int cmd, ...) {
+    FileHandle *fhc = get_fhc(fildes);
+    if (fhc == NULL) {
+        errno = EBADF;
+        return -1;
+    }
+
+    switch (cmd) {
+        case F_GETFL: {
+            int flags = 0;
+            if (fhc->is_blocking()) {
+                flags |= O_NONBLOCK;
+            }
+            return flags;
+        }
+        case F_SETFL: {
+            va_list ap;
+            va_start(ap, cmd);
+            int flags = va_arg(ap, int);
+            va_end(ap);
+            int ret = fhc->set_blocking(flags & O_NONBLOCK);
+            if (ret < 0) {
+                errno = -ret;
+                return -1;
+            }
+            return 0;
+        }
+
+        default: {
+            errno = EINVAL;
+            return -1;
+        }
+    }
+}
+
 extern "C" int poll(struct pollfd fds[], nfds_t nfds, int timeout)
 {
     if (nfds > OPEN_MAX) {

platform/mbed_retarget.h

@@ -43,14 +43,20 @@ typedef unsigned int  uid_t;    ///< User ID
 typedef unsigned int  gid_t;    ///< Group ID
 #endif
 
-#define O_RDONLY 0      ///< Open for reading
-#define O_WRONLY 1      ///< Open for writing
-#define O_RDWR   2      ///< Open for reading and writing
-#define O_CREAT  0x0200 ///< Create file if it does not exist
-#define O_TRUNC  0x0400 ///< Truncate file to zero length
-#define O_EXCL   0x0800 ///< Fail if file exists
-#define O_APPEND 0x0008 ///< Set file offset to end of file prior to each write
-#define O_BINARY 0x8000 ///< Open file in binary mode
+/* Flags for open() and fcntl(GETFL/SETFL)
+ * At present, fcntl only supports reading and writing O_NONBLOCK
+ */
+#define O_RDONLY 0        ///< Open for reading
+#define O_WRONLY 1        ///< Open for writing
+#define O_RDWR   2        ///< Open for reading and writing
+#define O_NONBLOCK 0x0004 ///< Non-blocking mode
+#define O_APPEND   0x0008 ///< Set file offset to end of file prior to each write
+#define O_CREAT    0x0200 ///< Create file if it does not exist
+#define O_TRUNC    0x0400 ///< Truncate file to zero length
+#define O_EXCL     0x0800 ///< Fail if file exists
+#define O_BINARY   0x8000 ///< Open file in binary mode
+
+#define O_ACCMODE   (O_RDONLY|O_WRONLY|O_RDWR)
 
 #define NAME_MAX 255    ///< Maximum size of a name in a file path
 
@@ -480,6 +486,10 @@ enum {
     DT_SOCK,    ///< This is a UNIX domain socket.
 };
 
+/* fcntl.h defines */
+#define F_GETFL 3
+#define F_SETFL 4
+
 struct pollfd {
     int fd;
     short events;
@@ -504,6 +514,7 @@ extern "C" {
     int isatty(int fildes);
     int fsync(int fildes);
     int fstat(int fildes, struct stat *st);
+    int fcntl(int fildes, int cmd, ...);
     int poll(struct pollfd fds[], nfds_t nfds, int timeout);
     int close(int fildes);
     int stat(const char *path, struct stat *st);