DIDFT_AXIS is a generic mask for absolute and relative axes.

Implemented ntdll's RtlInitializeSListHead, RtlQueryDepthSList,
RtlInterlockedFlushSList, RtlFirstEntrySList,
RtlInterlockedPushEntrySList, RtlInterlockedPopEntrySList, their
kernel32 equivalents, and tests.

Reported by Misha Koshelev.

rasapi32: RasEnumConnections now returns correct buffer size (zero) when there are zero connections available.

gdi32: Make sure that we don't read/write beyond the provided buffer in GetOutlineTextMetricsA, add a test case.