The threee tags `<name>`, `</name>` and `<command>` were frequently used
with a leading space that this removes. The reason this habbit is so
widespread in testcases is probably that they have been copy and pasted.
Hence, fixing them all now might curb this practice from now on.
Closes#12028
For pop3/imap/smtp, added test 891 to somewhat verify the pop3
case.
For this, I enhanced the pingpong test server to be able to send back
responses with LF-only instead of always using CRLF.
Closes#2150