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Conflicts:
	ChangeLog
	src/lib/dhcp/cfgmgr.h
	src/lib/dhcp/tests/cfgmgr_unittest.cc

Conflicts:
	ChangeLog
	src/lib/dhcp/tests/Makefile.am

committing at my discretion.

Sunstudio is unhappy with plain NULL inside EXPECT_EQ and thinks it is
int. This makes it think otherwise.

This should fix compilation issue on sunstudio.

Planned for merge #2268

this works around a bit awkward installation setup where there's a link
from "python3" to "python3.x" but not from "python3-config" to the
corresponding "python3.x-config".  That happens for recent versions of
Homebrew.

And make it slightly stronger (it now checks something that should be
mostly obvious too, but who knows, with software).

Instead of releasing it directly. While the internal release was more
convenient, it didn't allow for swapping things fast under a mutex and
then spending the time releasing it unlocked.

It is supposed to be mostly private. Use it internally only.

as the new code actually doesn't do rollback operation internally,
even though the observable effect is the same.

the previous one was not really accurate and was confusing.