configure checks if you have lcov & genhtml; these can be specified
with --with-lcov and/or --with-genhtml.

If they are available, the make target coverage can be executed:

- make clean, then make check with CC=gcc and CFLAGS set so that
  inputs for gcov/lcov are generated.

- lcov creates the src/coverage subdir and generates a targetfile
  there.

- genhtml generates HTML reports in src/coverage.

Also change the date outputs to include time zones.

Led to some bugfixing in the VFP code (generating proper error
messages).

Also fix a memory leak (after removing an addr_ent from the free list).

By using BASE64URLNOPAD from VMOD blob.

After the derive_prenonce() call, the local array cek, which had
the right value after the prior call to derive_cek(), was apparently
overwritten. So copy it before that can happen.

memcmp() is defined to return 0 when the length param is 0.

Changes in the keys are expected to be rare compared to key reads.
But when there are changes, they should be executed urgently (a
key may need to be changed because it has been compromised).

We are no longer using hard-wired keys.