Auto merge of #3199 - devnexen:redox_getdtablesize, r=JohnTitor

adding getdtablesize on redox

Author: bors

libc-test/semver/redox.txt

@@ -194,6 +194,7 @@ getrlimit
 getrusage
 getservbyport
 getservent
+getdtablesize
 killpg
 lockf
 madvise

src/unix/redox/mod.rs

@@ -993,6 +993,7 @@ extern "C" {
 
     // unistd.h
     pub fn pipe2(fds: *mut ::c_int, flags: ::c_int) -> ::c_int;
+    pub fn getdtablesize() -> ::c_int;
 
     // malloc.h
     pub fn memalign(align: ::size_t, size: ::size_t) -> *mut ::c_void;