Only raw errno codes are logged if FileCommit fails. These are
implementation-specific, so it makes it harder to debug based on
user reports. Instead, use SysErrorString to display both the
raw int value and the descriptive message.
These files change infrequently, and not much header shuffling is required.
We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.
Some uses of non-threadsafe `strerror` have snuck into the code since
they were removed in #4152. Add a wrapper `SysErrorString` for
thread-safe strerror alternatives and replace all uses of `strerror`
with this.
