Prepare to move the airpdcap code to epan/crypt (SVN won't let me actually

Prepare to move the airpdcap code to epan/crypt (SVN won't let me actually
move the files until these changes are checked in).  Add an AC_DEFINE
for airpdcap (which will be removed once the changes have settled).
Update the airpdcap code to compile on non-Windows systems.  Fix up
comments and whitespace to conform more closely to the rest of the
code base.  Verified to compile under Windows and OS X.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@20227 f5534014-38df-0310-8fa8-9805f1628bb7