gawk -i inplace is an order of magnitude faster when also redirecting stdout

[Ed Morton]

Someone on StackExchange was asking about their gawk script being slow 
and someone else (https://unix.stackexchange.com/a/771263/133219) 
pointed out that using `-i inplace` is an order of magnitude slower if 
you don't also redirect stdout which seems unintuitive at best.

For example given a 1 million line input file created by:

   $ seq 1000000 > file1m

and using:

   $ awk --version
   GNU Awk 5.3.0, API 4.0, PMA Avon 8-g1, (GNU MPFR 4.2.1, GNU MP 6.3.0)

If we just reproduce it as-is using `-i inplace` the timing is:

   $ time { awk -i inplace '1' file1m; }

   real    0m2.544s
   user    0m0.265s
   sys     0m1.843s

whereas if we redirect stdout even though there is no stdout produced:

   $ time { awk -i inplace '1' file1m >/dev/null; }

   real    0m0.236s
   user    0m0.187s
   sys     0m0.000s

As you can see that second execution with stdout redirected ran an order 
of magnitude faster. The person who investigated thinks it's due to the 
first execution being considered "interactive" since stdout isn't 
technically being redirected and so doing line buffering vs the second 
execution being "non-interactive" due to stdout being redirected and so 
using a larger buffer.

If that is the case, could gawk be updated to consider "inplace" editing 
as non-interactive? If not, I think it'd be worth a statement in the 
manual about this difference in performance between the 2.

    Ed.