security: update /proc restriction mechanism

NixOS option `security.hideProcessInformation` for globally restricting
access to /proc has been removed.
Use per-service restrictions via 'ProtectProc' instead.

Rename
`nix-bitcoin.security.hideProcessInformation` to
`nix-bitcoin.security.dbusHideProcessInformation`
because this option now only implements the dbus restriction.

test/tests.py

@@ -85,9 +85,6 @@ def _():
     succeed('[[ $(stat -c "%U:%G %a" /secrets/dummy) = "root:root 440" ]]')
 
     if "secure-node" in enabled_tests:
-        # Access to '/proc' should be restricted
-        machine.succeed("grep -Fq hidepid=2 /proc/mounts")
-
         machine.wait_for_unit("bitcoind")
         # `systemctl status` run by unprivileged users shouldn't leak cgroup info
         assert_matches(
@@ -97,7 +94,6 @@ def _():
         # The 'operator' with group 'proc' has full access
         assert_full_match("runuser -u operator -- systemctl status bitcoind 2>&1 >/dev/null", "")
 
-
 @test("bitcoind")
 def _():
     assert_running("bitcoind")