cygwin: export __getpagesize

The inclusion of <sys/cygwin.h> by <sys/shm.h>, besides causing namespace
pollution, also makes it very difficult to get the WINVER-dependent parts
of the former.  This affects code (such as x11vnc -unixpw_nis) which use
both SysV shared memory (e.g. the X11 MIT-SHM extension) and user password
authentication.

getpagesize is the simplest function to retreive this information, but it
is a legacy function and would also pollute the global namespace. The LSB
lists another form which is in the implementation-reserved namespace:

http://refspecs.linuxfoundation.org/LSB_3.1.0/LSB-Core-generic/LSB-Core-generic/baselib---getpagesize.html

Signed-off-by: Yaakov Selkowitz <yselkowi@redhat.com>

winsup/cygwin/common.din

@@ -60,6 +60,7 @@ __freading NOSIGFE
 __fsetlocking SIGFE
 __fwritable NOSIGFE
 __fwriting NOSIGFE
+__getpagesize = getpagesize SIGFE
 __getreent NOSIGFE
 __gnu_basename NOSIGFE
 __infinity NOSIGFE

winsup/cygwin/include/cygwin/shm.h

@@ -13,7 +13,6 @@ details. */
 #define _CYGWIN_SHM_H
 
 #include <cygwin/ipc.h>
-#include <sys/cygwin.h>
 
 #ifdef __cplusplus
 extern "C"
@@ -24,7 +23,9 @@ extern "C"
  *
  * 64 Kb was hardcoded for x86. MS states this may change so the constant
  * expression is replaced by a function call returning the correct value. */
-#define SHMLBA	(cygwin_internal (CW_GET_SHMLBA))
+#define SHMLBA	(__getpagesize ())
+/* internal alias of legacy getpagesize to avoid polluting global namespace */
+int __getpagesize (void);
 
 /* Shared memory operation flags:
  */

winsup/cygwin/include/cygwin/version.h

@@ -477,13 +477,14 @@ details. */
       293: Convert utmpname/utmpxname to int.
       294: Export clog10, clog10f.
       295: Export POSIX ACL functions.
+      296: Export __getpagesize.
      */
 
      /* Note that we forgot to bump the api for ualarm, strtoll, strtoull,
 	sigaltstack, sethostname. */
 
 #define CYGWIN_VERSION_API_MAJOR 0
-#define CYGWIN_VERSION_API_MINOR 295
+#define CYGWIN_VERSION_API_MINOR 296
 
      /* There is also a compatibity version number associated with the
 	shared memory regions.  It is incremented when incompatible

winsup/cygwin/shm.cc

@@ -21,6 +21,10 @@ details. */
 #include "sync.h"
 #include "ntdll.h"
 
+/* __getpagesize is only available from libcygwin.a */
+#undef SHMLBA
+#define SHMLBA (wincap.allocation_granularity ())
+
 /*
  * client_request_shm Constructors
  */