Security hardening (#426)

- Switch gid and supplementary groups to user's when reading user's
  config
- Only show users between UID_MIN and UID_MAX in /etc/login.defs
- Open accountsservice icons with O_NOFOLLOW to explicitly disallow
  symlinks

Cargo.lock

@@ -1366,6 +1366,7 @@ dependencies = [
  "tracing",
  "tracing-journald",
  "tracing-subscriber",
+ "whitespace-conf",
  "xdg 3.0.0",
  "zbus 5.11.0",
 ]
@@ -7368,6 +7369,12 @@ dependencies = [
  "web-sys",
 ]
 
+[[package]]
+name = "whitespace-conf"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88bce969a15062afb2d34155402d78cd50d1ddeca000a42353ef0c71ddd68995"
+
 [[package]]
 name = "widestring"
 version = "1.2.0"