log: Use generalized sr_log() to implement logging helpers

log: Use generalized sr_log() to implement logging helpers

Get rid of the specicialized sr_err(), sr_warn(), etc. functions.
Instead, define the logging helper macros in terms of sr_log(),
and remove the sr_log() helper macro so that no function is hidden
by a macro anymore.

Decorate sr_log() with G_GNUC_PRINTF to detect varargs errors. This
unearthed a gazillion warnings all over the place which will have
to be fixed.

Also convert the helper macros to ISO C99 __VA_ARGS__ style instead
of relying on a GNU C extension. Paste the log prefix directly into
the format string to make this work.