isatty: Make it return true in Cygwin consoles on native Windows.
* lib/isatty.c: Include <string.h>.
(GetProcAddress): New macro.
(GetNamedPipeClientProcessIdFuncType): New type.
(GetNamedPipeClientProcessIdFunc): New variable.
(QueryFullProcessImageNameFuncType): New type.
(QueryFullProcessImageNameFunc): New variable.
(initialized): New variable.
(initialize): New function.
(IsCygwinConsoleHandle): New function.
(isatty): Invoke it.
* doc/posix-functions/isatty.texi: Mention the issue.