Add new --level-prefix option

This prepends a severity level such as <3> to each line of diagnostic
output, with numeric severity levels taken from matching syslog(3)
(such as LOG_ERR = 3), so that the diagnostic output can be parsed by
tools like `logger --prio-prefix` and `systemd-cat --level-prefix=1`
that support that encoding.

The facility (LOG_USER, etc.) is not included, since it makes little
sense to vary on a per-message basis. logger(1) supports prefixes
with or without a facility, and systemd-cat(1) only supports prefixes
without a facility, so this is compatible with both.

A future version of Steam's pressure-vessel is likely to use this to
make warnings and fatal errors from bubblewrap more visible.

Signed-off-by: Simon McVittie <smcv@collabora.com>

utils.h

@@ -53,6 +53,8 @@ typedef int bool;
 #define PR_SET_CHILD_SUBREAPER 36
 #endif
 
+extern bool bwrap_level_prefix;
+
 void  bwrap_log (int severity,
                  const char *format,
                  ...) __attribute__((format (printf, 2, 3)));