env-file: bring our decoding of double-quoted strings in env files in line with shell

env-file: bring our decoding of double-quoted strings in env files in line with shell

In shell, inside of double quotes only a select few chars should be
escaped. If other chars are escaped this has no effect. Correct the list
of chars that need such escaping.

Also, make sure we can read back the stuff we wrote out without loss.

Fixes: #16788