For just a handful of people, rbenv init would hang indefinitely. Turning on
debugging output suggested that the `read` expression to split PATH into a bash
array was hanging, but I could never reproduce this myself. Instead, this uses
bash v4+ `readarray` if it's available, or falls back to bash v3 `read` with the
default DELIM being a newline character. This will cause a regression if any PATH
entries contain a literal newline character, but hopefully people are not relying
on such paths.
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