Cygwin: uname: Raise size of utsname fields and revamp uname(2) output

New format:

  sysname:      CYGWIN_NT-${osversion}-${os_build_number}[-WOW64]
  nodename:     `gethostname`
  release:      ${cygwin_version}-${API minor}.${arch}[.snap]
  version:      YYYY-MM-DD HH:MM UTC
  machine:      ${arch}
_GNU_SOURCE:
  domainname:   `getdomainname`
!_GNU_SOURCE:
  __domainname: `getdomainname`

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/wincap.h

@@ -59,6 +59,7 @@ public:
   const size_t allocation_granularity () const
 		     { return (size_t) system_info.dwAllocationGranularity; }
   const char *osname () const { return osnam; }
+  const DWORD build_number () const { return version.dwBuildNumber; }
   const bool is_wow64 () const { return !!wow64; }
 
 #define IMPLEMENT(cap) cap() const { return ((wincaps *) this->caps)->cap; }