Allow Tor to be compiled without support for relay mode

Let's make some more optional modules.

Our target set of modules might include:

dirauth - the code only used by directory authorities (including bridge authorities)
dircache - the code only used by directory caches and directory authorities
relay - the code only used by relays and directory authorities
common - the code used by all roles

I'll do a design, and a proposed CI build strategy, and then get it reviewed.

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