Avoid using unions for LongDirEntry.

GCC still assumes that the dirent has no data past the end for some
scenarios here and still mis-optimizes things. Therefore, drop the
usages of unions altogether, and instead use a casted character array.

Additionally, use B_FILE_NAME_LENGTH for the array, not B_PATH_NAME_LENGTH,
and make sure to add 1 for the NULL terminator.

Storage: Rework LongDirEntry to be a union.

Our dirent structure is "slim": it has a flexible-length array at the
end which must be allocated to whatever size the consumer wants. However,
we use [1] there and not [0] or [], which meant GCC thought it was not
a flexible-length array, and so it optimized various string accesses
that it assumed must be always false. Among these was BDirectory's
check for "." and "..", and so that resulted in infinite loops.

When changing our dirent structure to a proper FLA instead of [1],
GCC then throws errors on LongDirEntry as it has data "after" the
FLA; which is what we want, but there is no way to tell GCC that.
So now we use a union instead, which is the proper way to statically
allocate a FLA.

This is part of #17389, but the real fix requires changing our dirent
structure, which is coming in a separate commit.

MergedDirectory: missing include

BMergedDirectory::GetNextDirents(): fix end-of-list return value

Add private class BMergedDirectory to storage kit

An arbitrary number of directories can be added, which the implemented
BEntryList interface presents as a single merged entry list. Three
different merge policies are supported which define how entries that
appear in more than one directory are treated.

MergedDirectory: missing include

BMergedDirectory::GetNextDirents(): fix end-of-list return value

Add private class BMergedDirectory to storage kit

An arbitrary number of directories can be added, which the implemented
BEntryList interface presents as a single merged entry list. Three
different merge policies are supported which define how entries that
appear in more than one directory are treated.