Before this commit, many debugger `where` functions transformed the
filename of the current debugger location to a path relative to vim's
current working directory. When the user changed the current working
directory in the meantime, the previously saved location could not
resolved any more and lead to errors. This commit fixes this problem by
storing absolute paths instead.
Mdbg already had this feature, since the Windows SDK doesn't normally
put it in the PATH(just like any other Microsoft tool that assumes you
are going to use Visual Studio and don't need the PATH...)
Only supports setting variables and executing functions, because that's
what GDB supports(via different commands)(I think - didn't find anything
else...)
