Fix creation of Exec path for files not in prefix

Fix creation of Exec path for files not in prefix

Doing strcat() into a static buffer produces incorrect results for
the second and subsequent services if they are not in the ${prefix};
for example, if the first call should have returned
"C:\bar\bin\service1" and the second should have returned
"C:\bar\bin\service2", the second result would actually be
"C:\bar\bin\service1C:\bar\bin\service2".

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=83539
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
[smcv: added commit message; used strncpy/strncat to avoid overflow]
Reviewed-by: Ralf Habacker <ralf.habacker@freenet.de>