Fix configure script build with autoconf < 2.63

We added some AS_VAR_IF-based checks to detect whether we have
managed to compile (but not link) with stack-protector.  On autoconf
before 2.63, we don't have AS_VAR_IF, so we just have to let the
user get a compile error rather than a helpful "find libssp" error.

Fixes bug 12693; bugfix on 0.2.5.2-alpha (commit 21ac292820da578)
New API for policies_parse_exit_policy().
Using the new API in unit-test.
Whitespace fixes
Adding changes file for 8197.
Writing comments for newly added functions.
Merge branch 'bug8197_squashed'

Conflicts:
	src/test/test_policy.c
Merge branch 'master' of git-rw.torproject.org:/tor
Merge remote-tracking branch 'public/bug12693_025'