More pg_test_fsync fixups.

More pg_test_fsync fixups.

Reduce #includes to minimum actually needed; in particular include
postgres_fe.h not postgres.h, so as to stop build failures on some
platforms.

Use get_progname() instead of hardwired program name; improve error
checking for command line syntax; bring error messages into line with
style guidelines; include strerror result in die() cases.