gpg: Detect already compressed data also when using a pipe.
ce8ffd71b724
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Description

gpg: Detect already compressed data also when using a pipe.

* common/iobuf.c (file_filter_ctx_t): Add fields for the peek feature.
(file_filter): Implement peeking.
(iobuf_ioctl): Add new IOBUF_IOCTL_PEEK.
* common/iobuf.h (IOBUF_IOCTL_PEEK, IOBUFCTRL_PEEK): New.
* common/miscellaneous.c (is_file_compressed): Rewrite.  Detect PDF.
* g10/encrypt.c (encrypt_simple): Peek before detecting compression.
(encrypt_crypt): Ditto.
* g10/sign.c (sign_file): Also detect already compressed data.
* g10/options.h (opt): Add explicit_compress_option.
* g10/gpg.c (main): Set opt.explicit_compress_option for -z.

Note that this patch also introduces a compression check for signing
which was never done in the past.

Backported-from-master: 60963d98cfd8e60f88ee43c2d992f6dd3bbbd74c

Note that sign.c (sign_file) has been re-indented to ease future
backports.