The Prism header is in theory variable-length, but in practice it's

The Prism header is in theory variable-length, but in practice it's
always 144 bytes long.  However, some drivers on Linux use
ARPHRD_IEEE80211_PRISM, but sometimes or always supply an AVS header, so
we have to check whether the radio header is a Prism header or an AVS
header, so, in practice, it's variable-length.

Treat DLT_PRISM_HEADER as having a variable-length header, and generate
code to find the length of the Prism header that first checks for an AVS
header and, if we have an AVS header, gets the length from the header,
and otherwise just gets a length of 144.  This fixes Sourceforge bug
1847574.

Sort various references to the radio headers (case labels, functions,
etc.) into the same order (Prism, AVS, radiotap), for consistency.  Put
PPI after them all.

Handle 802.11 and 802.11-plus-radio-header with a common case when
initializing.