Logging macros and improving error/warning messages

[kmk3]

Background

This PR was opened to improve errExit() messages:

• modif: improve errExit error messages #5871

However, errExit is mostly only used when basic libc calls fail (as it uses
perror(3p) to print the errno message); most warning/error messages are
printed manually and do not contain the file path. Example:

Error: invalid sandbox name

Proposal

So how about adding more logging macros?

Examples:

log_trace("starting foo");
// TRACE src/firejail/util.c:123 x(): starting foo

log_debug("foo is %d, foo);
// DEBUG src/firejail/util.c:123 check_foo(): foo is 1

log_info("Reading profile %s", profile);
log_info("firejail version 0.9.73");
// INFO  profile_read(): Reading profile /etc/firejail/foo.profile
// INFO  profile_read(): Reading profile /etc/firejail/globals.local
// [...]
// INFO  main(): firejail version 0.9.73

log_warning("failed to open file: %s", foo);
// WARN  src/firejail/util.c:123 check_foo(): failed to open file: /foo/bar

log_error("invalid --foo: %s", foo);
// ERROR src/firejail/main.c:123 main(): invalid --foo: aaa-
// (program exits)

log_fatal("malloc"); // this is basically errExit()
// FATAL src/firejail/util.c:123 check_foo(): malloc: Cannot allocate memory
// (program exits)

Main differences between functions:

• log_info does not include the file path (for readability)
• log_error and log_fatal exit the program
• log_fatal uses perror(3p) to print the reason (like errExit)

I'm not sure about the format of info and below, so the first steps would be to
replace manual warning/error messages (fprintf calls) with the log functions
so that they include the file path and function in the message; how the
functions operate internally (format, destination, etc) could be improved
later.

Also add --loglevel= to control which messages get printed:

• --loglevel=[trace|debug|info|warning|error|fatal]
• --quiet would map to --loglevel=error
• --debug would map to --loglevel=debug or --loglevel=trace

The default would be info.

As for the last item, considering the amount of things that --debug prints
(such as the assembly dumps of seccomp filters), it seems more like a trace
option, so maybe it would make sense to change the messages that are currently
inside if (arg_debug) to use log_trace() and then deprecate --debug.

Relates to:

• Add visual cue for firejail outputs #2322
• WIP: improve firejail's error messaging #3325
• Option --quiet suppresses important warnings #3354
• Where to print debug messages (stdout or stderr)? #3749
• Rework quiet, debug and normal console output #4275
• modif: improve errExit error messages #5871