Support (or not) for "Large" file mode

[jphickey]

The CFDP specification (specifically CCSDS 727.0-B-5, table 5-1) defines a "Large File Flag" bit. When this bit is set in the PDU header, it means that all file sizes and offsets are 64 bits in size, rather than the historical 32 bits.

The CF implementation currently does not have this bit defined - the position it resides is defined as RESERVED in this code - and all file sizes are defined as a fixed 32 bits in the PDU structures, with no provisions to permit use of 64 bit values here.

The CF requirements also do not appear to require large file transfer to be supported.

However, even if large file is not supported by this implementation of CFDP, for interoperability reasons CF should at least recognize and check for this flag being set, and reject packets for which it is set. Due to the fact that this flag changes the size and location of elements that follow, if it is ignored then one runs the risk of misinterpreting packets and triggering undefined behavior, which could corrupt the filesystem.

[skliper]

Somewhat related to nasa/osal#657 on the transmit side where OS_lseek silently truncates OS results larger than 32 bit.

[jphickey]

Similar to #129 here, this should check if the bit is set and reject packets for which it is set. In this case encode/decode routines are still hardcoded to 32-bit file sizes, so CF still has a bit interpretation error if this is ever set, even after PR #137.