xzgrep: Fix escaping of malicious filenames (ZDI-CAN-16587).

Malicious filenames can make xzgrep to write to arbitrary files
or (with a GNU sed extension) lead to arbitrary code execution.

xzgrep from XZ Utils versions up to and including 5.2.5 are
affected. 5.3.1alpha and 5.3.2alpha are affected as well.
This patch works for all of them.

This bug was inherited from gzip's zgrep. gzip 1.12 includes
a fix for zgrep.

The issue with the old sed script is that with multiple newlines,
the N-command will read the second line of input, then the
s-commands will be skipped because it's not the end of the
file yet, then a new sed cycle starts and the pattern space
is printed and emptied. So only the last line or two get escaped.

One way to fix this would be to read all lines into the pattern
space first. However, the included fix is even simpler: All lines
except the last line get a backslash appended at the end. To ensure
that shell command substitution doesn't eat a possible trailing
newline, a colon is appended to the filename before escaping.
The colon is later used to separate the filename from the grep
output so it is fine to add it here instead of a few lines later.

The old code also wasn't POSIX compliant as it used \n in the
replacement section of the s-command. Using \<newline> is the
POSIX compatible method.

LC_ALL=C was added to the two critical sed commands. POSIX sed
manual recommends it when using sed to manipulate pathnames
because in other locales invalid multibyte sequences might
cause issues with some sed implementations. In case of GNU sed,
these particular sed scripts wouldn't have such problems but some
other scripts could have, see:

    info '(sed)Locale Considerations'

This vulnerability was discovered by:
cleemy desu wayo working with Trend Micro Zero Day Initiative

Thanks to Jim Meyering and Paul Eggert discussing the different
ways to fix this and for coordinating the patch release schedule
with gzip.

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