Auto merge of #1487 - lpetre:add_more_utime_constants, r=gnzlbg

Adding UTIME_NOW and UTIME_OMIT to dragonfly

Verified in both repos:
 - [musl](https://git.musl-libc.org/cgit/musl/tree/include/sys/stat.h#n70)
 - [dragonfly](e7ab884bd4/sys/sys/stat.h (L250))

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

@@ -1002,6 +1002,10 @@ pub const SF_NOHISTORY: ::c_ulong = 0x00400000;
 pub const SF_CACHE:     ::c_ulong = 0x00800000;
 pub const SF_XLINK:     ::c_ulong = 0x01000000;
 
+// timespec constants
+pub const UTIME_OMIT: c_long = -2;
+pub const UTIME_NOW: c_long = -1;
+
 fn _CMSG_ALIGN(n: usize) -> usize {
     (n + 3) & !3
 }