Throughout, remove "include <errno.h>" from files which already include

cygerrno.h.
* include/cygwin/config.h (__DYNAMIC_REENT__): Define.
* include/cygwin/version.h: Bump API minor version.
* cygwin.din: Export __getreent
* cygerrno.h: Include errno.h.  Fix places where _impure_ptr is used directly
to store the errno value.
* debug.cc (__set_errno): Ditto.
* errno.cc: Remove _RRENT_ONLY define to get errno.cc compiled.
* signal.cc: Rename _reent_clib to _REENT throughout.
* thread.h (reent_clib): Remove prototype.
* thread.cc (reent_clib): Rename reent_clib to __getreent.  Return _impure_ptr
until MTinterface is initialized.
(reent_winsup): Fix a possible SEGV when _r == NULL.  Return NULL instead.
* MTinterface::fixup_after_fork: Switch reent back to _impure_ptr to keep
signal handling running when fork is called from a thread other than the
mainthread.

winsup/cygwin/include/cygwin/version.h

@@ -207,12 +207,13 @@ details. */
            exported symbols like fopen64, which might confuse configure.
        86: Export ftok
        87: Export vsyslog
+       88: Export _getreent
      */
 
      /* Note that we forgot to bump the api for ualarm, strtoll, strtoull */
 
 #define CYGWIN_VERSION_API_MAJOR 0
-#define CYGWIN_VERSION_API_MINOR 87
+#define CYGWIN_VERSION_API_MINOR 88
 
      /* There is also a compatibity version number associated with the
 	shared memory regions.  It is incremented when incompatible