The inotify interface has an unfortunate side effect of making Mutt
react too quickly to new mail.  Sometimes, the mail is only
half-delivered when the mailbox is checked.  Because Mutt is using the
stat mtime - seconds resolution - this means it won't realize there
are more messages delivered during the same second.

Nanosecond resolution fields were standardized in POSIX.1-2008, so
check for and use those if they are available.