Auto merge of #1474 - lpetre:add_utime_constants, r=gnzlbg

Adding UTIME_NOW and UTIME_OMIT to OSes which support utimensat

I've managed to verify a handful of these:

- [FreeBSD](https://github.com/freebsd/freebsd/blob/1d6e4247415d264485ee94b59fdbc12e0c566fd0/tools/build/stat.h#L35)
- [NetBSD](https://github.com/NetBSD/src/blob/64b8a48e1288eb3902ed73113d157af50b2ec596/sys/sys/stat.h#L235)
 - [Apple](https://opensource.apple.com/source/xnu/xnu-4903.221.2/bsd/sys/stat.h.auto.html)

I'm less confident in these:
 - [Haiku](https://github.com/haiku/haiku/blob/abb59d7351c7ddb50c63c40430a82d94fa61917a/headers/posix/sys/stat.h#L105)
   - I could be wrong on this one: https://github.com/haiku/haiku/search?q=UTIME_NOW&unscoped_q=UTIME_NOW
 - [WASI](https://github.com/CraneStation/wasi-libc/blob/24792713d7e31cf593d7e19b943ef0c3aa26ef63/libc-top-half/musl/include/sys/stat.h#L71)
  - I could be wrong on this one: https://github.com/CraneStation/wasi-libc/search?q=UTIME_NOW&unscoped_q=UTIME_NOW
 - [Solarish](https://github.com/illumos/illumos-gate/blob/4e0c5eff9af325c80994e9527b7cb8b3a1ffd1d4/usr/src/uts/common/sys/stat.h#L478)