comm/libminimal.la to facilitate helper use of convenience libs (#1009)

comm/libminimal.la to facilitate helper use of convenience libs (#1009)

Helpers may not (want to) use fd_open() and fd_close() directly, but
they (want to) use libdebug which does use fd_open() and fd_close().

Currently, the Comm "importing" fd_open() and Comm "delisting"
fd_close() APIs are not implemented inside src/comm/ but they do belong
to the Comm module and use its internals. Moving fd.h to src/comm/ will
require a lot of noisy out-of-scope changes deserving a dedicated PR.

Despite their names, the minimally-implemented functions do not open(2)
and close(2) file descriptors in their full implementations either: That
full implementation just updates fd-associated Squid-specific metadata
that Squid helpers do not need/use.

Do not use stub_fd.cc in the deployed/non-test pinger program.

Avoid build-breaking copying of stub_fd.cc source file in test-suite/.