After a few hours of watching strange things happening I finally stumbled
on the cause.
Very short summary: the postgres binary needs to be linked to libpthread,
as this will ensure a special fork() inside libthr (the FreeBSD libpthread
implementation/wrapper) is used which correctly deals with any pthread
usage (I am guessing especially statically initialized pthread_X variables
in for example libxml2).
This can be accomplished by adding "-lpthread" to the LDFLAGS when building
either libxml2 or postgresql itself, or by preloading libpthread.so when
starting postgresql.
Before advocating any particular solution, I'd like to throw this out first
and see how strongly people feel about this.
Cheers, and thanks to everyone who replied so far
Dennis