cgi_decode_string: do not crash if the string contains a malformed escape sequence.
cgi_decode_string: do not crash if the string contains a malformed escape sequence.

Also, decode strings that contain percent signs in the output (like %2525)
correctly, i.e. unescape only once.

mapmeld/fortran-machine,mapmeld/fortran-machine,mapmeld/fortran-machine,mapmeld/fortran-machine,mapmeld/fortran-machine