gpg,common: Make sure that all fd given are valid.

* common/sysutils.c (gnupg_fd_valid): New function.
* common/sysutils.h (gnupg_fd_valid): New declaration.
* common/logging.c (log_set_file): Use the new function.
* g10/cpr.c (set_status_fd): Likewise.
* g10/gpg.c (main): Likewise.
* g10/keylist.c (read_sessionkey_from_fd): Likewise.
* g10/passphrase.c (set_attrib_fd): Likewise.
* tests/openpgp/Makefile.am (XTESTS): Add the new test.
* tests/openpgp/issue2941.scm: New file.

Consider a situation where the user passes "--status-fd 3" but file
descriptor 3 is not open.

During the course of executing the rest of the commands, it's possible
that gpg itself will open some files, and file descriptor 3 will get
allocated.

In this situation, the status information will be appended directly to
whatever file happens to have landed on fd 3 (the trustdb? the
keyring?).

This is a potential data destruction issue for all writable file
descriptor options:

--status-fd
--attribute-fd
--logger-fd

It's also a potential issue for readable file descriptor options, but
the risk is merely weird behavior, and not data corruption:

--override-session-key-fd
--passphrase-fd
--command-fd

Fixes this by checking whether the fd is valid early on before using
it.

• GnuPG-bug-id: T2941
• Signed-off-by: Justus Winter <justus@g10code.com>