Removed missing/dup2.c

This function should be always available, as POSIX-compliant or
Windows platform are required since 1.9.  Also the code in this
file is MT-unsafe.

LEGAL

@@ -531,7 +531,6 @@ mentioned below.
 [include/ruby/st.h]
 [missing/acosh.c]
 [missing/alloca.c]
-[missing/dup2.c]
 [missing/erf.c]
 [missing/finite.c]
 [missing/hypot.c]

configure.ac

@@ -1858,10 +1858,16 @@ AS_CASE(["$target_os"],[freebsd*],[
 	 AC_REPLACE_FUNCS(close)
 	 ])
 
+AC_DEFUN([RUBY_REQUIRE_FUNC], [
+    AC_CHECK_FUNCS([$1])
+    AS_IF([test "$ac_cv_func_[]AS_TR_SH($1)" = yes], [],
+          [AC_MSG_ERROR($1[() must be supported])])
+])
+m4_map_args_w([dup dup2], [RUBY_REQUIRE_FUNC(], [)])
+
 AC_REPLACE_FUNCS(acosh)
 AC_REPLACE_FUNCS(cbrt)
 AC_REPLACE_FUNCS(crypt)
-AC_REPLACE_FUNCS(dup2)
 AC_REPLACE_FUNCS(erf)
 AC_REPLACE_FUNCS(explicit_bzero)
 AC_REPLACE_FUNCS(ffs)
@@ -1922,7 +1928,6 @@ AC_CHECK_FUNCS(dirfd)
 AC_CHECK_FUNCS(dl_iterate_phdr)
 AC_CHECK_FUNCS(dlopen)
 AC_CHECK_FUNCS(dladdr)
-AC_CHECK_FUNCS(dup)
 AC_CHECK_FUNCS(dup3)
 AC_CHECK_FUNCS(eaccess)
 AC_CHECK_FUNCS(endgrent)

include/ruby/missing.h

@@ -84,10 +84,6 @@ RUBY_EXTERN double atanh(double);
 RUBY_EXTERN char *crypt(const char *, const char *);
 #endif
 
-#ifndef HAVE_DUP2
-RUBY_EXTERN int dup2(int, int);
-#endif
-
 #ifndef HAVE_EACCESS
 RUBY_EXTERN int eaccess(const char*, int);
 #endif

io.c

@@ -462,16 +462,14 @@ rb_cloexec_fcntl_dupfd(int fd, int minfd)
     }
 #elif defined(HAVE_FCNTL) && defined(F_DUPFD)
     ret = fcntl(fd, F_DUPFD, minfd);
-#elif defined(HAVE_DUP)
+#else
     ret = dup(fd);
     if (ret >= 0 && ret < minfd) {
         const int prev_fd = ret;
         ret = rb_cloexec_fcntl_dupfd(fd, minfd);
         close(prev_fd);
     }
     return ret;
-#else
-# error "dup() or fcntl(F_DUPFD) must be supported."
 #endif
     if (ret < 0) return ret;
     rb_maygvl_fd_fix_cloexec(ret);