config: Add file size and nanosecond resolution fields to the cached modified config file information.

Repeatedly modifying config files and reloading too fast sometimes fails
to reload the configuration because the cached modification timestamp has
one second resolution.

* Added file size and nanosecond resolution fields to the cached config
file modification timestamp information.  Now if the file size changes or
the file system supports nanosecond resolution the modified file has a
better chance of being detected for reload.

* Added a missing unlock in an off-nominal code path.

(closes issue AST-1303)

Review: https://reviewboard.asterisk.org/r/3235/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@408387 f38db490-d61c-443f-a65b-d21fe96a405b