Allow trivial-validate.py to validate single file

If the first argument to trivial-validate is an existing file,
then only run validation tests on that file. Optionally, specify
a directory in which to check for duplicates of this file.
Ex: python trivial-validate.py <filename> <rulesets-directory-name>

Otherwise, if the argument is a directory, default to previous
behavior.
don't chdir if there is only one file to test
Merge branch '3.0' of git://git.torproject.org/git/https-everywhere into 3.0
[Dell] mixed content; also linux.dell.com redirects to http

https://trac.torproject.org/projects/tor/ticket/8569