fix compiler errors -O3

* "dereferencing type-punned pointer will break
strict-aliasing rules"
* missing initializer
* boundary
Add support for libpcap immediate mode

New flag of USE_PCAP_IMMEDIATE_MODE was introduced.
Please add -DUSE_PCAP_IMMEDIATE_MODE in the compiler arguments to
use immediate mode.
Merge pull request #107 from iHiroakiKawai/fix_compile

fix compiler errors -O3
Merge pull request #108 from iHiroakiKawai/libpcap_immediate

Add support for libpcap immediate mode
fix warnings