conmon: Use strerror(errno) instead of %m
conmon: Use strerror(errno) instead of %m

Avoid:

  $ make clean && make CFLAGS=-Wpedantic 2>&1 | head -n5
  rm -f conmon.o cmsg.o ../bin/conmon
  cc -Wpedantic -std=c99 -Os -Wall -Wextra -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include   -c -o conmon.o conmon.c
  conmon.c: In function ‘write_k8s_log’:
  conmon.c:32:19: warning: ISO C does not support the ‘%m’ gnu_printf format [-Wformat=]
     fprintf(stderr, "[conmon:e]: %s %m\n", s);     \
                     ^

from printf(3) [1]:

  m (Glibc extension; supported by uClibc and musl.)  Print output of
    strerror(errno).  No argument is required.

strerror, on the other hand, is in POSIX [2].

[1]: http://man7.org/linux/man-pages/man3/printf.3.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/strerror.html

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

vbatts/cri-o,kubernetes-incubator/ocid,rhatdan/cri-o,kubernetes-incubator/cri-o,mikebrow/cri-o,jwhonce/ocid,jwhonce/ocid,kubernetes-incubator/cri-o,vbatts/cri-o,mikebrow/cri-o,kubernetes-incubator/cri-o,nalind/cri-o,kubernetes-incubator/cri-o,kubernetes-incubator/ocid,vbatts/cri-o,vbatts/cri-o,mikebrow/cri-o,kubernetes-incubator/ocid,nalind/cri-o,rhatdan/cri-o,jwhonce/ocid,mikebrow/cri-o,rhatdan/cri-o,nalind/cri-o,rhatdan/cri-o