Likewise, it's useful to provide an alternate value during an
environment expansion, if the environment variable isn't already set.
For instance, $LD_LIBRARY_PATH will inadvertently search the current
working directory if it starts or ends with a colon, so the following
is usually wrong:
LD_LIBRARY_PATH=/foo/lib:${LD_LIBRARY_PATH}
To address that, this changes replace_env to support the following
shell compatible alternate value syntax: