Improve our clang-format.sh script

[nmathewson]

Previously, this script ran over every C file in our source code,
which took up to a minute on my desktop.

Instead, the script now has several modes that it can run in, to
minimize the time spent and improve useful output. It should now be
suitable for everyday use and git hooks. I hope.

I've also renamed this script, so that we can keep using it in the
future if we were to move to some tool other than clang-format.

[coveralls]

Pull Request Test Coverage Report for Build 9091

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 179 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.01%) to 64.316%

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Files with Coverage Reduction	New Missed Lines	%
src/feature/hs/hs_common.c	1	84.48%
src/feature/client/entrynodes.c	178	86.3%

Totals
Change from base Build 9083:	0.01%
Covered Lines:	51133
Relevant Lines:	79503

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💛 - Coveralls