Deduplication of pcap header read procedures. (#2082)

* Deduplicate reading of pcap header code.

* Move toString to be free function.

* Update tests + Lint

* Fix needs swap being written to on empty file.

* Enforce strict packing.

* Lint

* Try pre-checking the remaining bytes in stream before read to fix Mac CI fail.

* Fixup checks

* Add safe version of buffer comparison macro that accepts lengths for both buffers.

* Update buffer comparisons.

* Move error checking code to append mode block.

* Add debug statements.

* Explicitly set read pointer to start of file on append mode.

* Lint

* Revert writePcapHeader to be inside PcapFileWriterDevice.

* Remove else.

Author: Dimi1010

Pcap++/header/PcapFileDevice.h

@@ -306,44 +306,13 @@ namespace pcpp
 		void flush();
 
 	private:
+		static bool writeHeader(std::ostream& outStream, FileTimestampPrecision precision, uint32_t snaplen,
+		                        LinkLayerType linkType);
+
 		LinkLayerType m_PcapLinkLayerType = LINKTYPE_ETHERNET;
 		bool m_NeedsSwap = false;
 		FileTimestampPrecision m_Precision = FileTimestampPrecision::Unknown;
 		std::fstream m_PcapFile;
-
-		struct CheckHeaderResult
-		{
-			enum class Result
-			{
-				HeaderOk,
-				HeaderError,
-				HeaderNeeded
-			};
-
-			Result result;
-			std::string error;
-			bool needsSwap = false;
-
-			static CheckHeaderResult fromOk(bool needsSwap)
-			{
-				return { Result::HeaderOk, "", needsSwap };
-			}
-
-			static CheckHeaderResult fromError(const std::string& error)
-			{
-				return { Result::HeaderError, error };
-			}
-
-			static CheckHeaderResult fromHeaderNeeded()
-			{
-				return { Result::HeaderNeeded };
-			}
-		};
-
-		static bool writeHeader(std::fstream& pcapFile, FileTimestampPrecision precision, uint32_t snaplen,
-		                        LinkLayerType linkType);
-		static CheckHeaderResult checkHeader(std::fstream& pcapFile, FileTimestampPrecision requestedPrecision,
-		                                     LinkLayerType requestedLinkType);
 	};
 
 	/// @class PcapNgFileReaderDevice