Fixed local timezone was incorrectly being applied to IAM and Last-Modified dates.

This issue was causing last modified file times to show incorrectly in cases where the last modified time was retrieved from the "Last-Modified" header. This can happen when files are added to a bucket outside of s3fs and do not have the "x-amz-meta-mtime" header.

`timegm` should be used to convert to `time_t` in UTC, rather than `mktime`.