x/sys/unix: add *at (POSIX.1-2008 conformed) functions on darwin.

[hirochachacha]

OS X 10.10 (IOS 8.0) add list of *at syscalls.

• openat
• renameat
• fchmodat
• fstatat
• mkdirat
• faccessat
• fchownat
• linkat
• readlinkat
• symlinkat
• unlinkat

Currently, only fchmodat is exposed.

[perillo]

These syscalls are also supported on FreeBSD, and I need them for a project that currently only supports Linux.

[avg-I]

I would like to second both requests (darwin / FreeBSD) in this issue.
Other popular BSDs (Net, Open)most likely support the requested system calls as well.

[calmh]

Yes. Openat is required, as I understand it, to be able to reliably open a path without following symlinks. Currently this is only possible on Linux, with Go...

[ianlancetaylor]

I encourage anyone who wants to fix this to write a patch and submit it following the guidelines at https://golang.org/doc/contribute.html.

[gopherbot]

CL https://golang.org/cl/50450 mentions this issue.