safety: add longitudinal pre-enable state for brake at a standstill

[sshane]

Adds new pre-enable state when enabling at a standstill with brake depressed. Allows brake and negative accel to 0 accel, blocks positive accel, gas, interceptor.

Pre-reqs (so this change isn't unmaintainable):

• [x] https://github.com/commaai/panda/pull/1171
• [x] https://github.com/commaai/panda/pull/1168
• [x] https://github.com/commaai/panda/pull/1178
• [x] https://github.com/commaai/panda/pull/1179
• [x] https://github.com/commaai/panda/pull/1176
• [x] Common angle-based steer checks