userlock was a contrib module providing advisory locking functionality. It was removed in PostgreSQL 8.2 (commit af7d257e) following the integration of functions providing equivalent functionality into core PostgreSQL.
The userlock module appears to have been introduced in PostgreSQL 6.3 as part of commit 951986c5. Following removal in PostgreSQL 8.2, the code was moved to the now defunct pgFoundry but evidently received no further attention.
See the commit log for the change history of this module within the PostgreSQL source tree.