vmspawn,journal-remote: add journal forwarding disk usage options

Add options to vmspawn to configure journal-remote disk usage limits
when forwarding journal entries from the VM. These are passed through
as --max-use=, --keep-free=, --max-file-size=, and --max-files=
command-line arguments to systemd-journal-remote.

Add --max-use=, --keep-free=, --max-file-size=, and --max-files=
command-line options to systemd-journal-remote to allow overriding the
corresponding settings from the configuration file.

Add $SYSTEMD_JOURNAL_REMOTE_CONFIG_FILE environment variable support
to systemd-journal-remote. When set, the specified file is used
instead of the default configuration file and drop-in directories.
When set to the empty string or /dev/null, configuration file parsing
is skipped entirely. vmspawn sets this to /dev/null in the child
process to avoid inheriting the host's journal-remote configuration.

Make fork_notify() argv parameter optional. When NULL is passed,
fork_notify() returns 0 in the child (with $NOTIFY_SOCKET set) and
lets the caller run custom code before exec. Returns 1 in the parent.
This allows vmspawn to set environment variables in the child without
polluting the parent process.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

shell-completion/bash/systemd-vmspawn

@@ -38,7 +38,7 @@ _systemd_vmspawn() {
         [BIND]='--bind --bind-ro'
         [SSH_KEY]='--ssh-key'
         [CONSOLE]='--console'
-        [ARG]='--cpus --ram --vsock-cid -M --machine --uuid --private-users --background --set-credential --load-credential'
+        [ARG]='--cpus --ram --vsock-cid -M --machine --uuid --private-users --background --set-credential --load-credential --forward-journal-max-use --forward-journal-keep-free --forward-journal-max-file-size --forward-journal-max-files'
         [IMAGE_FORMAT]='--image-format'
         [IMAGE_DISK_TYPE]='--image-disk-type'
     )