Modernize by replacing foreach with ranged-for.

Use QRandomGenerator for Qt >= 5.15.

Compiling with Qt5.15 gives a warning that `qrand` is deprecated. It suggests using `QRandomGenerator` instead, which is done here.

Only include PTR records that correspond to the query target

This avoids creating massive PTR queries containing all PTR records ever received

Fix new/delete mismatch caught by ASAN.

Fix the Resolver example in the docs to use the correct name

Correct QMDNSENGINE_EXPORT when BUILD_SHARED_LIBS is undefined

Fix SIGSEGV on macOS