Improved config_chain parsing by using escape for \ " and ' (close #1952)

Improved config_chain parsing by using escape for \ " and ' (close #1952)

It also add checks against failed malloc.

The option value should be escaped by \ for the mentionned characters and
only for them (and only one time).

For example
 dst="test \"ok\".mp3"
will assign the value
 test "ok".mp3
to the option dst.

The following one
 dst="c:\test\\'bla'bla.txt"
will assign the value
 c:\test\'bla'bla.txt

You can use the functions
- config_StringEscape (allocates memory)
- config_StringUnescape (does not allocate memory).