Fix --brute for p_lx_elf.cpp, which compresses multiple pieces

Force all pieces to use the same de-compressor.
(Future: allow each PT_LOAD to choose its own.)
Has minor wobbles due to page alignment, and size of de-compressor.
64-bit only for now.
https://github.com/upx/upx/issues/570
https://github.com/upx/upx/issues/297
	modified:   p_lx_elf.cpp
	modified:   p_unix.cpp
	modified:   packer.cpp
	modified:   packer.h

src/packer.h

@@ -341,6 +341,10 @@ private:
     Packer &operator=(const Packer &) = delete;
 };
 
+int     force_method(int method); // (0x80ul<<24)|method
+int    forced_method(int method); // (0x80ul<<24)|method ==> method
+int is_forced_method(int method); // predicate
+
 #endif /* already included */
 
 /* vim:set ts=4 sw=4 et: */