POP3: fix multi-line responses

Some POP3 commands are multi-line, e.g. have responses terminated by a
last line with '.', but some are not. Define the known command
properties and fi

POP3: fix multi-line responses

Some POP3 commands are multi-line, e.g. have responses terminated by a
last line with '.', but some are not. Define the known command
properties and fix response handling.

Add test case for STAT.

Fixes #14677
Reported-by: ralfjunker on github
Closes #14707

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