Fix mtime option in file.find

While the docstring indicates that a +/- modifier is accepted for the
mtime argument to file.find, the regex that parses the mtime expression
does not match a leading plus or minus. This commit modifies that regex,
and then uses the +/- modifier to determine whether to match files older
or newer than the difference indicated by the mtime expression.

Fixes #2828.
Merge pull request #7471 from terminalmage/issue2828

Fix mtime option in file.find