[patch] add if_selinux_enabled attribute, make it the default

[Havoc Pennington]

Hi,

Looks good (with your fix for including system-local.conf). Also, can
you add an include statement with this attribute to the test suite.
(test/data/valid-config-files)
You could even get fancy and test error paths by adding invalid config
files with stuff like if_selinux_enabled="notaboolean"

Thanks,
Havoc