Guarantee LLProcess::Params::postend listener any ReadPipe data.

Previously one might get process-terminated notification but still have to
wait for the child process's final data to arrive on one or more ReadPipes.
That required complex consumer timing logic to handle incomplete pending
ReadPipe data, e.g. a partial last line with no terminating newline. New code
guarantees that by the time LLProcess sends process-terminated notification,
all pending pipe data will have been buffered in ReadPipes.
Document LLProcess::ReadPipe::getPump() notification event; add "eof" key.
Add LLProcess::ReadPipe::getline() and read() convenience methods.
Add static LLProcess::getline() and basename() convenience methods, publishing
logic already present elsewhere.
Use ReadPipe::getline() and read() in unit tests.
Add unit test for "eof" event on ReadPipe::getPump().
Add unit test verifying that final data have been buffered by termination
notification event.