Provide convenience routines for manipulating domainsets in the kernel.

domainset_create() will take a key, validate it, and return a valid
domainset that can be placed on an object or other.

sysctl_handle_domainset() allows manipulation of policy from sysctl. Use
this to make a default vnode domainset.

Clean-up some header creep. Define a generic bitset type. Rewrite some
string functions to be more generic.