util/init_random_seed: write_file implies deterministic

This makes Zeek run in deterministic mode with --save-seeds usage
and reworks all the extra indirections used in init_random_seed()
to make it easier to follow the control flow.

Fixes #4209

testing/btest/Baseline/core.save-load-seeds/load.conn.log.cut

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+uid	history	service
+CHhAvVGS1DHFjwGM9	ShADadFf	http

testing/btest/Baseline/core.save-load-seeds/save.conn.log.cut

@@ -0,0 +1,3 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+uid	history	service
+CHhAvVGS1DHFjwGM9	ShADadFf	http

testing/btest/core/save-load-seeds.zeek

@@ -0,0 +1,16 @@
+# @TEST-DOC: Save seeds and read and assure the UIDs are the same. Regression test for #4209
+#
+# @TEST-EXEC: zeek --save-seeds myseeds -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: mkdir save && mv *log save
+# @TEST-EXEC: zeek-cut -m uid history service < save/conn.log >save/conn.log.cut
+#
+# @TEST-EXEC: zeek --load-seeds myseeds -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: mkdir load && mv *log load
+# @TEST-EXEC: zeek-cut -m uid history service < load/conn.log >load/conn.log.cut
+#
+# @TEST-EXEC: btest-diff load/conn.log.cut
+# @TEST-EXEC: btest-diff save/conn.log.cut
+# @TEST-EXEC: diff load/conn.log.cut save/conn.log.cut
+
+@load base/protocols/conn
+@load base/protocols/http