Clean up signed vs. unsigned.

Clean up signed vs. unsigned.

Have separate cpack_ routines for signed and unsigned numbers, with the
signed ones using _S_ extract macros.  That way, we can do more type
checking.

Add EXTRACT_LE_S_ macros.

Use signed variables for IEEE80211_RADIOTAP_TX_ATTENUATION and
IEEE80211_RADIOTAP_DB_TX_ATTENUATION, rather than using unsigned
variables that we cast to int.

Also, use EXTRACT_U_1() in cpack_uint8.