In anticipation of adding settings and stats.

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.