comsat: Don't read arbitrary files

When processing a notification, instead of accepting any file name
that doesn't begin with a slash, accept only file names that don't
contain any slashes at all. This makes it possible to notify a
user about a mailbox that doesn't bear their name, as long as they
are permitted to read it, but prevents comsat from reading files
outside the mail spool.

PR: 270404
MFC after: 1 week
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D51580