gpg: Allow for positional parameters in the passphrase prompt.

* g10/passphrase.c (passphrase_get): Replace sprintf by xasprintf.
--

Without that at least the French translation does not always work
because it requires positional parameters.  Windows for example does
not support them as they are not defined by C99 but by POSIX.
build: Print an error message if zlib is not installed.

* configure.ac (missing_zlib): New.
gpg: Need to init the trustdb for import.

* g10/trustdb.c (clear_ownertrusts): Init trustdb.

--

This was fixed in 1.4 branch in commit
23191d7851eae2217ecdac6484349849a24fd94a but was not applied to the
2.0 branch that exhibits the same problem. This is actually a hack
to fix a bug introduced with commit 2528178.

GnuPG-bug-id: 1622