awk: bring in vendor branch from upstream 20210727

Changes since the last import:

July 27, 2021:
As per IEEE Std 1003.1-2008, -F "str" is now consistent with
-v FS="str" when str is null. Thanks to Warner Losh.

July 24, 2021:
Fix readrec's definition of a record. This fixes an issue
with NetBSD's RS regular expression support that can cause
an infinite read loop. Thanks to Miguel Pineiro Jr.

Fix regular expression RS ^-anchoring. RS ^-anchoring needs to
know if it is reading the first record of a file. This change
restores a missing line that was overlooked when porting NetBSD's
RS regex functionality. Thanks to Miguel Pineiro Jr.

Fix size computation in replace_repeat() for special case
REPEAT_WITH_Q. Thanks to Todd C. Miller.

Also, for the first time, import all the tests.

Sponsored by: Netflix