log: split long messages before sending them to syslog()

It is known that syslog() has a size limitation, and long messages
won't make it to the syslog daemon. In order to avoid that, and so
that syslog() calls don´t end up with invalid reads reported by
valgrind, just limit the size of the log lines sent to some sane value
like 512, and make sure the long log entries (e.g. QMI or MBIM message
taces) get split before sending them.

  ModemManager[123834]: <debug> [1661440355.646049] [/dev/cdc-wdm0] received message...
  <<<<<< RAW:
  <<<<<<   length = 173
  <<<<<<   data   = 01:AC:00:80:00:00:01:02:21:00:A1:00...

  ==123834== Invalid read of size 1
  ==123834==    at 0x533EE58: __vsyslog_internal (syslog.c:230)
  ==123834==    by 0x533F299: syslog (syslog.c:90)
  ==123834==    by 0x29CD7D: log_backend_syslog (mm-log.c:167)
  ==123834==    by 0x29D1A7: _mm_log (mm-log.c:265)
  ==123834==    by 0x29D21F: log_handler (mm-log.c:274)
  ==123834==    by 0x5080824: g_logv (gmessages.c:1387)
  ==123834==    by 0x5080AD3: g_log (gmessages.c:1456)
  ==123834==    by 0x4A77D61: trace_message.part.0 (qmi-device.c:1720)
  ==123834==    by 0x4A7969F: trace_message (qmi-device.c:1679)
  ==123834==    by 0x4A7969F: process_message (qmi-device.c:1811)
  ==123834==    by 0x4A7CEE8: qmi_endpoint_parse_buffer (qmi-endpoint.c:106)
  ==123834==    by 0x4A77982: endpoint_new_data_cb (qmi-device.c:1607)
  ==123834==    by 0x4FF5D33: UnknownInlinedFun (gclosure.c:893)
  ==123834==    by 0x4FF5D33: g_signal_emit_valist (gsignal.c:3406)
  ==123834==  Address 0x8a8568f is 1 bytes before a block of size 43 alloc'd
  ==123834==    at 0x4841888: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  ==123834==    by 0x533EDCC: __vsyslog_internal (syslog.c:206)
  ==123834==    by 0x533F299: syslog (syslog.c:90)
  ==123834==    by 0x29CD7D: log_backend_syslog (mm-log.c:167)
  ==123834==    by 0x29D1A7: _mm_log (mm-log.c:265)
  ==123834==    by 0x29D21F: log_handler (mm-log.c:274)
  ==123834==    by 0x5080824: g_logv (gmessages.c:1387)
  ==123834==    by 0x5080AD3: g_log (gmessages.c:1456)
  ==123834==    by 0x4A77D61: trace_message.part.0 (qmi-device.c:1720)
  ==123834==    by 0x4A7969F: trace_message (qmi-device.c:1679)
  ==123834==    by 0x4A7969F: process_message (qmi-device.c:1811)
  ==123834==    by 0x4A7CEE8: qmi_endpoint_parse_buffer (qmi-endpoint.c:106)
  ==123834==    by 0x4A77982: endpoint_new_data_cb (qmi-device.c:1607)
  ==123834==
  ==123834== Conditional jump or move depends on uninitialised value(s)
  ==123834==    at 0x4847D09: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  ==123834==    by 0x529A5C7: __vfprintf_internal (vfprintf-process-arg.c:397)
  ==123834==    by 0x52BA6F9: __vdprintf_internal (iovdprintf.c:54)
  ==123834==    by 0x528DD89: dprintf (dprintf.c:30)
  ==123834==    by 0x533EE82: __vsyslog_internal (syslog.c:230)
  ==123834==    by 0x533F299: syslog (syslog.c:90)
  ==123834==    by 0x29CD7D: log_backend_syslog (mm-log.c:167)
  ==123834==    by 0x29D1A7: _mm_log (mm-log.c:265)
  ==123834==    by 0x29D21F: log_handler (mm-log.c:274)
  ==123834==    by 0x5080824: g_logv (gmessages.c:1387)
  ==123834==    by 0x5080AD3: g_log (gmessages.c:1456)
  ==123834==    by 0x4A77D61: trace_message.part.0 (qmi-device.c:1720)