Auto merge of #1605 - zvirja:fix-mipsel-unknown-linux-uclibc, r=gnzlbg

Add ucred and socket related types to uclibc-mips32

Fix compilation of the [tokio](https://github.com/tokio-rs) rust for MIPS architecture when using uClibc. Added missed types, which are defined in library headers:
https://github.com/kraj/uClibc/blob/ca1c74d67dd115d059a875150e10b8560a9c35a8/libc/sysdeps/linux/common/bits/socket.h#L320
https://github.com/kraj/uClibc/blob/ca1c74d67dd115d059a875150e10b8560a9c35a8/libc/sysdeps/linux/common/bits/in.h#L35-L36
https://github.com/kraj/uClibc/blob/ca1c74d67dd115d059a875150e10b8560a9c35a8/libc/sysdeps/linux/common/bits/in.h#L140-L143

Should also solve deprecrated/net2-rs#88

Revisit of #1500

src/unix/uclibc/mips/mips32/mod.rs

@@ -220,6 +220,12 @@ s! {
         pub mem_unit: ::c_uint,
         pub _f: [::c_char; 8],
     }
+
+    pub struct ucred {
+        pub pid: ::pid_t,
+        pub uid: ::uid_t,
+        pub gid: ::gid_t,
+    }
 }
 
 pub const __SIZEOF_PTHREAD_CONDATTR_T: usize = 4;
@@ -587,6 +593,11 @@ pub const SYS_pwritev2: ::c_long = 4000 + 362;
 pub const SYS_pkey_mprotect: ::c_long = 4000 + 363;
 pub const SYS_pkey_alloc: ::c_long = 4000 + 364;
 pub const SYS_pkey_free: ::c_long = 4000 + 365;
+pub const IPV6_UNICAST_HOPS: ::c_int = 16;
+pub const IPV6_MULTICAST_IF: ::c_int = 17;
+pub const IPV6_MULTICAST_HOPS: ::c_int = 18;
+pub const IP_MULTICAST_IF: ::c_int = 32;
+pub const IP_MULTICAST_TTL: ::c_int = 33;
 
 #[link(name = "util")]
 extern "C" {