Auto merge of #2594 - devnexen:dfbsd_so_passcreds, r=Amanieu

dragonflybsd add SO_PASSCRED const.

Author: bors

libc-test/semver/dragonfly.txt

@@ -885,6 +885,7 @@ SOMAXOPT_SIZE
 SO_ACCEPTFILTER
 SO_CPUHINT
 SO_NOSIGPIPE
+SO_PASSCRED
 SO_SNDSPACE
 SO_TIMESTAMP
 SO_USELOOPBACK

src/unix/bsd/freebsdlike/dragonfly/mod.rs

@@ -964,6 +964,7 @@ pub const NOTE_CHILD: u32 = 0x00000004;
 
 pub const SO_SNDSPACE: ::c_int = 0x100a;
 pub const SO_CPUHINT: ::c_int = 0x1030;
+pub const SO_PASSCRED: ::c_int = 0x4000;
 
 pub const PT_FIRSTMACH: ::c_int = 32;