Add a coverage target

Run tests with coverage and report to coveralls.io

For simplicity, this currently only adds a single target in a
configuration that attempts to maximize coverage. The true CI coverage
from all the various builds may be a little larger.

The coverage run has the following configuration:
- no-asm: since we can't track asm coverage anyway, might as well measure the
  non-asm code coverage.
- Enable various disabled-by-default options:
  - rc5
  - md2
  - ec_nistp_64_gcc_128
  - ssl3
  - ssl3-method
  - weak-ssl-ciphers

Finally, observe that no-pic implies no-shared, and therefore running
both builds in the matrix is redundant.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Kurt Roeckx <kurt@openssl.org>