lib/gis: match prototype with declaration for G_strlcat and G_strlcpy

[nilason]

Make local implementations of strlcpy() and strlcat() optimizable by using restrict type qualifier, while keeping public API for G_strlcat and G_strlcpy available to C++ code and having identical prototype and declaration.

Currently the prototypes look like:

size_t G_strlcat(char *, const char *, size_t);

but declared as:

size_t G_strlcat(char *restrict dst, const char *restrict src, size_t dsize) { ...}

which the compilers seem to accept, but was probably not the best choice.

[wenzeslaus]

That makes sense and even somewhat looks like the other wrappers we have elsewhere!