add uc_read_fd function

include/ucore/read_file.h

@@ -21,6 +21,14 @@ extern "C" {
 char *
 uc_read_file(const char *file_name, size_t *length, size_t max);
 
+/** Read the content of a file descriptor as a stream.
+ * @see uc_read_file
+ * This is similar but reads from a file descriptor,
+ * of any type as long as it supports the read() call.
+ */
+char *
+uc_read_fd(int fd, size_t *length, size_t max);
+
 #ifdef __cplusplus
 }
 #endif