flux_security_configure() fails when there is no configuration

[grondo]

Currently flux_security_configure (sec, NULL) fails with a fatal error if there is no security TOML files in the default glob.

This means we currently have to ship a default config with the package, but this should not be necessary. Instead of flux-security is not configured it should use some sane and safe default.

[garlick]

Just hit this on a test system.

Yeah, if no config, the defaults should probably match our recommended config of

[sign]
max-ttl = 1209600  # 2 weeks
default-type = "munge"
allowed-types = [ "munge" ]