Improve SQUID_YESNO error messages (#1137)

Improve SQUID_YESNO error messages (#1137)

This macro performs a very specific validation check but was
passed a mix of different error messages to display on failure.
Many of which were unclear or wrong.

Move the text of the error inside the macro to enforce
consistency and accuracy about what failed. It actually only
needs to be passed the name of the ./configure options which
was configured badly.

Also, the options configured can only take the positive form
when they fail '--with-foo=X' or 'enable-foo=X' so update the
second argment to always show that form regardless of whether
the configure.ac logic has it default enabled or disabled.

Also, fixed a bug in --with-nat-devpf validation which was
testing the wrong variable (enableval). This bug only affects
the errors displayed so no builds were significantly affected.