Honor dos_header_t.e_cparhdr for small header

... and cleanup PackDjgpp2::readFileHeader()
https://github.com/upx/upx/issues/881

	modified:   p_djgpp2.cpp
	modified:   p_djgpp2.h

src/p_djgpp2.h

@@ -62,6 +62,16 @@ protected:
 
     unsigned coff_offset;
 
+    // based on https://dox.ipxe.org/PeImage_8h_source.html
+    struct alignas(1) dos_header_t {
+        LE16 e_magic;
+        LE16 e_cblp;    // Bytes on last page of file
+        LE16 e_cp;      // Pages in file
+        LE16 e_crlc;    // Relocations
+        LE16 e_cparhdr; // Size of header in paragraphs
+        LE16 e_OMITTED[32 - 5 * 2];
+    }; // 64 bytes
+
     struct alignas(1) external_scnhdr_t {
         byte _[12]; // name, paddr
         LE32 vaddr;