pg_waldump stucks with options --follow or -f and --stats or -z

Hi,

pg_waldump options, --follow or -f(to keep polling once per second for
new WAL to appear) and --stats or -z don't work well together i.e. the
command stucks [1]. I think the pg_waldump should emit an error. Note
that the pg_basebakup does error out on incompatible options.

Here's a small patch for fixing above along with a note in the documentation.

Thoughts?

[1] The following commands stuck:
./pg_waldump -p data/ -s 0/7000060 -f -z
./pg_waldump -p data/ -s 0/7000060 -f --stats='record'
./pg_waldump -p data/ -s 0/7000060 -f --stats='rmgr'

[2]
ubuntu3:~/postgres/inst/bin$ ./pg_waldump -p data/ -s 0/7000060 -f -z
pg_waldump: error: --follow and --stats are incompatible options
Try "pg_waldump --help" for more information.
bharath(at)bharathubuntu3:~/postgres/inst/bin$

Regards,
Bharath Rupireddy.