To avoid depending on the system default.

That way we can avoid using autopkgtest's 'allow-stderr' restriction.

It wasn't the case for interimap(1), see https://bugs.debian.org/608604 …
Fortunately we create $XDG_DATA_HOME/interimap with a secure mode, but
there is no reason to have the DB world-readable.  Since we can't rely
on SQLITE_OPEN_CREATE for secure mode we use sysopen(,,O_CREAT,0600).

And make the installation path configurable at `make` time.  Moreover,
adjust the 'test' target so the site directory and interimap/pullimap
path are configurable with INTERIMAP_I and INTERIMAP_PATH respectively.
That way one can run `tests/run foo` to check the source, `make test` to
check what's been built, and we also have the possibility to check the
installed program e.g. for autopkgtests.

And use security level 2 for ssl_cipher_list.  As of dovecot 2.3.18
ssl_min_protocol defaults to TLSv1.2.

This is required to test TLS version <1.2 on systems with higher
security levels, see SSL_CTX_set_security_level(3ssl).  Addapted from a
patch from <xnox> for Unbuntu.

Also, clarify that SSL_cipherlist only applies to TLSv1.2 and below.
See SSL_CTX_set_cipher_list(3ssl).