Remove broken PGXS code for pg_xlogdump

With the PGXS boilerplate in place, pg_xlogdump currently fails with an
ominous error message that certain targets cannot be built because
certain files do not exist.  Remove that and instead throw a quick error
message alerting the user of the actual problem, which should be easier
to diagnose that the statu quo.

Andres Freund
Add WaitForLockers in lmgr, refactoring index.c code

This is in support of a future REINDEX CONCURRENTLY feature.

Michael Paquier