unictype/property-byname: Reduce the number of load-time relocations.

* lib/unictype/pr_byname.c: Include <stdlib.h>.
(UC_PROPERTY_INDEX_*): New enumeration values.
(uc_property_byname): Convert an index from the lookup table to an
uc_property_t.
* lib/unictype/pr_byname.gperf: Store indices instead of uc_property_t
values.