builtin should be returned const

callers should never try to do any non-const operation with the
builtin public suffix list.
avoid the need for psl_global_init() and psl_global_deinit()

There is no need to for an initialization function if the builtin
structs if the comparison function will look at label_buf directly
when label == NULL.

This simplifies the API for users, who now don't have to worry about
library initialization and deinitialization functions (these sort of
functions can cause headaches in chained library loads, esp. in plugin
architectures like PAM).
Merge pull request #3 from dkg/master

API pruning