nall: Retrieve executable path more reliably

[jcm93]

In a sandboxed Flatpak environment on Linux, the current dladdr strategy for retrieving the binary path seems to provide the bare executable name ares for dli_fname rather than the binary's full location. There don't seem to be any guarantees about what sort of path (relative or absolute) will be provided for dli_fname, so we shouldn't rely on it.

On platforms where /proc/self/exe is available, we can use that in conjunction with readlink instead.

If /proc/self/exe is not available, we will fall back to the current dladdr strategy. readlink, meanwhile, should be available on any *nix platform supported by ares.

Note

Verified that this strategy works for retrieving the sandboxed binary path inside a Flatpak (/app/bin/ares), as intended.