Update expat to 2.2.6
Update contrib/expat by merging from the vendor branch.
Update expat_config.h manually, using
make -C /usr/ports/textproc/expat2 configure
as a baseline.
MFC after: 1 month
Relnotes: yes
Security: yes; see contrib/expat/Changes since 2.2.0
Sponsored by: Dell EMC Isilon