Use a tor_abort_() wrapper in our util_bug.h macros

[nmathewson]

Previously, our use of abort() would break anywhere that we didn't
include stdlib.h. This was especially troublesome in case where
tor_assert_nonfatal() was used with ALL_BUGS_ARE_FATAL, since that
one seldom gets tested.

As an alternative, we could have just made this header include
stdlib.h. But that seems bloaty.

Fixes bug 30189; bugfix on 0.3.4.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 4737

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.02%) to 60.301%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/lib/log/util_bug.c	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/feature/hs/hs_common.c	1	83.28%
src/feature/dirauth/shared_random.c	3	85.64%
src/feature/dirauth/dirvote.c	9	64.79%

Totals
Change from base Build 4665:	-0.02%
Covered Lines:	42857
Relevant Lines:	71072

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💛 - Coveralls