Update the lib/expat tree for the new v2.0.1 expat import. The bsdxml.h
header is now in two parts: bsdxml.h and bsdxml_external.h, representing
the expat.h and expat_external.h headers. Updated the info on the man
page as well. Also, fixed a type-error in a printf in
sbin/ifconfig/regdomain.c that would cause a compiler warning.

Approved by: sam, phk