Move safe_strerror to posix directory and the base namespace.

Since this is a wrapper around a posix thing that doesn't fall under any other categories, the posix directory is probably the best place.

Review URL: https://codereview.chromium.org/1160373004

Cr-Commit-Position: refs/heads/master@{#333524}
19 files changed