Prefix all cpack function names with "nd_". [skip ci]

Prefix all cpack function names with "nd_". [skip ci]

Gisle Vanem reported in GH #881 that in Windows tcpdump failed to link
with static AirPcap because the latter seems to include its own variant
of cpack code:

airpcap_static.lib(RadiotapDecode.obj) : error LNK2005: _cpack_init
already defined in cpack.obj
airpcap_static.lib(RadiotapDecode.obj) : error LNK2005: _cpack_uint16
already defined in cpack.obj
airpcap_static.lib(RadiotapDecode.obj) : error LNK2005: _cpack_uint32
already defined in cpack.obj
airpcap_static.lib(RadiotapDecode.obj) : error LNK2005: _cpack_uint64
already defined in cpack.obj
airpcap_static.lib(RadiotapDecode.obj) : error LNK2005: _cpack_uint8
already defined in cpack.obj

He confirms this change resolves the issue.