Replace `strlcpy` with `strncpy` in `msIO_stripStdoutBufferContentType()...

The use of `strlcpy` was producing *uninitialised value* errors in
valgrind.  It appears this was due to the fact that `strlcpy` expects
the source string to be null terminated which is not always the case
in this context.

The valgrind error was in the form:
```
Conditional jump or move depends on uninitialised value(s)
   at 0x5428601: strlcpy (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x543EDED: msIO_stripStdoutBufferContentType (in /usr/local/lib/libmapserver.so.6.3-dev)
   ...
 Uninitialised value was created by a heap allocation
   at 0x402BB7A: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
   by 0x543F0AF: msIO_bufferWrite (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x543E4C3: msIO_contextWrite (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x543E7A7: msIO_vfprintf (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x543E634: msIO_fprintf (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x543E2B6: msIO_setHeader (in /usr/local/lib/libmapserver.so.6.3-dev)
   by 0x53E9BAE: loadParams (in /usr/local/lib/libmapserver.so.6.3-dev)
   ...
```