util-lib: follow shell syntax for escape in quotes
Fixes #10659.
This changes the behaviour of parsing environment files to more closely
follow POSIX shell standards.
This has the effect that these variables defined in a file:
VAR1='\value'
VAR2="\value"
Are now interpreted as `\value` instead of interpreting the `\`
character and interpreting them as `value`.
For more information about the behaviour followed, see:
http://pubs.opengroup.org/onlinepubs/
9699919799/utilities/V3_chap02.html#tag_18_02