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