Update FileManager for use by SCL-F

[jmschonfeld]

There are two pieces of FileManager that need to be updated so that swift-corelibs-foundation can use this FileManager implementation:

1. When providing file attributes, many call-sites provide numeric attributes as SCL-F's NSNumber type. However, since swift-foundation does not know about NSNumber, it currently doesn't work. NSNumber is implicitly bridged to concrete numeric types, but not to an existential type like any BinaryInteger. So to fix this, when reading numeric attributes we now try a direct dynamic cast to the target type (as? T) which will succeed if value is an NSNumber, then we try a dynamic cast to any BinaryInteger to catch any other numeric types
2. User directories provided by XDG configuration files can be absolute or relative, but the current implementation assumes they are always relative. This change supports absolute paths.

[jmschonfeld]

@swift-ci please test