Fix the "dnssec" system test on Windows

Fix the "dnssec" system test on Windows

On Windows, the bin/tests/system/dnssec/signer/example.db.signed file
contains carriage return characters at the end of each line.  Remove
them before passing the aforementioned file to the awk script extracting
key IDs so that the latter can work properly.

(cherry picked from commit e4280ed9f561bc22e5c4f7817536d344598fcf3a)