MFV d1b143ee9a5: libbsdxml (expat) 2.4.6

Update contrib/expat from 2.4.3 to 2.4.6 to fix multiple vulnerabilities.

Reviewed by: delphij
MFC after: 2 weeks
Relnotes: yes
Security: yes; see contrib/expat/Changes since 2.4.3
Sponsored by: Dell EMC Isilon