[Exim] Low-priority wish for local_scan() API

Hello.

Philip,
Could you be so kind as to include string_cat() definition into the local_scan.h .

The function is quite helpful in constructing reply texts when
string_sprintf()'s functionality is not required, but as of exim-4.12 I had to
steal definition line from functions.h

I'm sure other authors of local_scan() functions would find it useful.

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./lxnt