This commit adds even more correct handling of parameters
with paths when they contain leading or trailing spaces and/or
slashes. Also it fixes problems that occur when the user specified
explicit paths to additional directories, but these paths match
the specified path of the data directory - in this case, additional
subdirectories should be treated (in relation to the data directory)
in the same way as if these paths were not specified or as if they
are implicitly specified as "." or "./". But prior to this fix,
existing code treated any values as if they were completely
separate directories, whether or not they actually point to the
same location to which datadir points to - and this sometimes
resulted in incorrect file transfers.

This fix does not contain separate tests, as tests will be
part of the main commit(s). This fix has been made as a separate
commit to facilitate review for major substantive fixes related
to MDEV-30157 and MDEV-28669.