Use a generic function to access the home directory making macOS path
access match other Unix systems.
When the function was added [0] it solved access to HOME when the
environment variable wasn't set. Since then support for `getpwuid()`
has been added on UNIX systems [1], which also works on macOS,
removing the need for macOS to have a separate function.
Furthermore BLI_expand_tilde had undocumented limitations that didn't
apply to other platforms (see PR for details).
Ref !128734
[0]: 9df13fba69
[1]: 6039cb17e6
cd9d4e992d