Bug #66407: POP client cannot handle long lines in message

https://savannah.nongnu.org/bugs/?66407 describes a case I
found in the POP client that fails on long lines.

As for the rest of nmh, I haven't seen any problems.  If I move a
message with a problematic image/jpeg part into an MH folder by
means other than POP, I can show and manipulate it.  Are there
specific tests people think I should try?

It does look like the POP client has built into it at several
levels the expectation that it can buffer an entire line.
For example, multiline() wants to unstuff doubled dots.

I see two possible approaches.  First, have netsec_readline be
willing to return partial lines, and return a new flag saying
whether it has done so.  Second, have multiline call netsec_read
(apparently unused anywhere else) and look for line endings
itself.  There are probably other approaches.  Thoughts?

 < Stephen