Read system PATHs on macOS.fish

# macOS: Read global PATH and MANPATH
function __read_system_path
    set -l var_name $argv[1]
    set -l filenames $argv[2..-1]
    set -x $var_name
    for filename in $filenames
        while read -l path
            set -x $var_name $$var_name $path
        end < $filename
    end
end
__read_system_path PATH /etc/paths /etc/paths.d/*
__read_system_path MANPATH /etc/manpaths /etc/manpaths.d/*
functions -e __read_system_path