Rewrite control_event_signal() to use signal_table.

[nmathewson]

When we added the ACTIVE and DORMANT virtual signals, we taught the
signal command to handle them, but we didn't teach SIGNAL event to
report them.

To solve this problem and prevent it from recurring, this patch
revises the implementation of control_event_signal() to use the same
signal_table that handle_control_signal() uses. This way, the two
controller commands can't become out of sync.

Fixes bug 33104; bugfix on 0.4.0.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 8002

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.007%) to 61.578%

---

Files with Coverage Reduction	New Missed Lines	%
src/feature/dirauth/shared_random.c	4	85.0%
src/feature/dirauth/dirvote.c	9	64.61%

Totals
Change from base Build 7914:	0.007%
Covered Lines:	47468
Relevant Lines:	77086

---

💛 - Coveralls