Add a libdnf5 plugin that acquires a systemd logind inhibitor lock
during package transactions to prevent system shutdown or reboot
while the system is in a critical state. The lock covers plugin hooks,
RPM transaction execution, system state updates, and history database
writes.

The plugin only implements pre_transaction() to acquire the lock. The
fd is registered with libdnf5::plugin::register_systemd_inhibitor_fd(),
a small standalone module that closes it unconditionally when the
transaction run finishes, regardless of which plugins are installed or
enabled - so the lock is guaranteed to be released even if this plugin
is later disabled or uninstalled after acquiring it. The mechanism is
keyed by the transaction object's address, so it requires no changes
to Transaction's own public interface.

The plugin config file is named 00-systemd-inhibit.conf so that it
sorts first alphabetically among all plugins, running before other
plugins' pre_transaction hooks.

Failure to acquire the lock is non-fatal: the transaction proceeds
normally with a warning logged.

Resolves: rpm-software-management#2408

Signed-off-by: Marek Blaha <mblaha@redhat.com>