Support absolute paths in RelativePathService

There are situations where it is difficult for a user to specify that a "path" attribute's value is absolute when used in conjunction with a "relative-to" attribute; specifically when the relative-to attribute has a default value (e.g. jboss.server.data.dir). In these situations, the xsd should state that "path" will only be treated as relative if it is not an absolute path (i.e. starts with '/', '
' or 'x:\' where 'x' is any character in the range [a-z] or [A-Z]). In these situations, RelativePathService should check if the 'path' value is absolute, and if it is, delegate to AbsolutePathService, ignoring any 'relative-to' value.

In cases where 'relative-to' has no default, the existing behavior should be preserved; the presence or absence of 'relative-to' controls whether 'path' is treated as absolute, and including an absolute path in cojunction with a 'relative-to' value is a user error.