Calculates true parent's relative path in Differencing VHD locator

[maxpat78]

This patch is tested both in Windows 11 and Linux (Ubuntu, WSL). It refines the Differencing VHD handler in following ways:

• always tries to handle an absolute W2ku locator;
• more log messages to debug;
• 2nd locator offset is relative to 1st, a fixed position is no more assumed;
• calculates and stores the true parent's relative path in W2ru locator (parent's path has to be relative to child's path, not current working directory).

As a consequence, VHD sets created in Windows can be shared with Linux users and vice-versa (due to correct relative locator).

NOTE FOR THE WILLING PROGRAMMER: a -relink switch could be added to VHDMAKE allowing to move a parent VHD and manually relink its childs.

Closes #5145

[maxpat78]

Test for Linux user:

• prepare a subdir and a subdir/subsubdir testing dirs in working directory;
• run following batch file in DosBox-X shell:

imgmake subdir/parent.vhd -t vhd -size 100 -force
vhdmake -f -l subdir/parent.vhd subdir/subsubdir/child.vhd 
imgmount d subdir/subsubdir/child.vhd
imgmount -u d

[maron2000]

Do we need to ignore these files?
There are no such file in the current code tree.

[maxpat78]

No, you don't. In fact those files came by mistake from another code tree of mine, but they hadn't to.