fix: multi-prefix replacement in binary files

[wolfv]

The pkg-config package encodes (in the binary) a "PATH" variable that looks something like:

/prefix_placeholder/lib:/prefix_placeholder/etc/share\0

So far we only replaced the initial prefix placeholder, and skipped past the rest.

With this change we first get the whole "c-string" (until the \0), and then replace each occurence of the prefix in the c-string. The padding is then added to make sure that the strings are of the same length.