do_child(): Take parameters via stdin lines, instead of command line

arguments, to avoid exposing addresses to shell.

Protocol:

 line 1: batchnum
 line 2: sender
 line 3...n+2: n recipients
 line n+3: <empty> - delimiting end of recipients
 line n+4: message content

ContactTransport(): Similarly, pass parameters to contact_transport in
the same manner, via contact_transport process stdin.

This avoids both security problems (and address like `touch
/etc/passwd`, as someone on the developers list warned) and distortion
problems (shell metacharacters in addresses).

Additionally, instrumented both the main process and the child process
with a stderr logger, distinguishing the labels for the two so we can
tell them apart in the error logs.

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