gstinfo: Rework stack trace detection a bit

gstinfo: Rework stack trace detection a bit

Ensure that the code paths for HAVE_UNWIND and HAVE_DBGHELP are never
taken at the same time, even if the build file code changes.

Prefer DbgHelp over libunwind on Windows in case both are somehow
available because DbgHelp is only available when building with the
MSVC toolchain, and libunwind won't give us debug symbols from objects
built with the MSVC toolchain.

Also, print slightly more useful messages for the level of stack trace
support enabled, and document what each if conditional does.