Expand test coverage support to entire tree

Test coverage support now covers the entire source tree, including
contrib, instead of just src/backend.  In a related but independent
development, the commands make coverage and make coverage-html can be run
in any directory.

This turned out to be much easier than feared.  Besides a few ad hoc fixes
to pass the make target down the tree, change all affected makefiles to
list their directories in the SUBDIRS variable, changed from variants like
DIRS and WANTED_DIRS.  MSVC build fix was attempted as well.