Optimize xpns_normalize_directory and xpns_is_valid_directory

[ReDTerraN]

xpns_normalize_directory() - instead of calling $_dir variable and change it three times, we just call it once and print it together with /#~/$HOME

xpns_is_valid_directory() - small clean up to improve readability, removing the else to instead use a NOT operator to check if the directory is there or not.

Fixed typo in one comment.

As before, manual testing and smokescreen testing has been done on the code.
But please check so I have not made any mistake.

Cheers

[greymd]

Thanks!