Fix unresolved reference when using optimization flag (-O2) against musl

[Guiglin]

In case you want to fortify headers you get errors like this:

/usr/include/fortify/unistd.h:156:1: error: reference to overloaded function could not be resolved; did you mean to call it?
  156 | _FORTIFY_FN(readlink) ssize_t readlink(const char *__p,

This is due of the readlink function from osutils.h that conflicts with readlink from unistd.h. To avoid that I propose to simply rename string readlink to string readsymlink.
To test, just add -O2 in the src/Makefile to CXXFLAGS