GetProgPath() now resolves the path derived from argv[0] always
via realpath(3p) to its real location in the filesystem, and returns
the path including binary name, effectively making it work like
windows' GetModuleFileName(), so we can re-use the existing code
to strip the trailing binary name.
since symlinks are now always resolved, we no longer need the
special case code for linux to use /proc/self/exe for this purpose.
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