Fix recently-introduced expand-file-name bug

Fix recently-introduced expand-file-name bug

The bug was that (expand-file-name "~") returned something
like "/home/eggert/" instead of "/home/eggert".
Problem	reported by Mattias Engdegård (Bug#26911#27).
* src/fileio.c (Fexpand_file_name): When concatenating NEWDIR to
NM, instead of stripping trailing slashes from NEWDIR (which can
turn non-symlinks into symlinks), strip leading slashes from NM.
This also simplifies the code by removing no-longer-needed DOS_NT
special-casing.  Also, remove an unnecessary ‘target[length] = 0;’
as that byte will be overwritten by the next memcpy anyway.
* test/src/fileio-tests.el (fileio-tests--HOME-trailing-slash):
New test.