[6.0] Prevent null character in Windows home directory

[jrflat]

Explanation: Prevents a null-terminator from the Windows home directory API from ending up in the resulting String or URL. Prevents %00 from being decoded to \0 when getting a URL's fileSystemPath, which is often converted to a C string for file system calls.
Scope: Windows-only change from String(decoding:) to String(decodingCString:) for homeDirectoryPath(forUser:). Not decoding %00 occurs for file paths of URLs on all platforms.
Original PR: #808
Risk: Low. Minor changes to prevent unexpected behavior when null-characters are included in file paths. The respective Windows APIs null-terminate their output, so String(decodingCString:) is safe.
Testing: swift-ci testing and local testing on Windows
Reviewers: @compnerd @jmschonfeld

[jrflat]

@swift-ci please test