fix fopen FILE* unsigned comparison bug

[pnfisher]

iotc_bsp_io_fs_open() in

src/bsp/platform/posix/iotc_bsp_io_fs_posix.c

sets resource_handle_out to the FILE* returned by fopen() if fopen() doesn't fail. This in turn gets assigned to the resource manager's context->resource_handle. Later, iotc_resource_manager_read() in

src/libiotc/io/fs/iotc_resource_manager.c

checks to make sure context->resource_handle is not less the zero. But there's nothing about fopen() that ensures that the FILE* returned by fopen() can't be a value less than zero (when used in a signed comparison).

If iotc_resource_manager_read() is going to assume a value less than 0 indicates a BSP file io open error, then we need to replace the use of fopen() in iotc_bsp_io_fs_posix.c with open(). #72

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[pnfisher]

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