lib: remove CONSTCOND comment

Since 2021-01-31, lint doesn't need it anymore for the common pattern of
'do ... while (0)'.

fix print-like attrs

A little more restructuring so that we don't need mutex stuff in syslog_ss.

Split syslog.c to:

- syslog_ss.c *_ss api functions (don't use stdio, time)
- syslog.c: *syslog* non _ss api functions (use stdio, time)
- xsyslog.c> common guts.

The motivation for this is not to

Split syslog.c to:

- syslog_ss.c *_ss api functions (don't use stdio, time)
- syslog.c: *syslog* non _ss api functions (use stdio, time)
- xsyslog.c> common guts.

The motivation for this is not to drag in stdio/locale/floating point/time
for every binary, since syslog_ss() is used in __stack_check_fail() for SSP.

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