pgsql: Clean up ecpg's use of mmerror(): const-ify the format argument,

Log Message:
-----------
Clean up ecpg's use of mmerror(): const-ify the format argument, add an
__attribute__() marker so that gcc can validate the format string against
the actual arguments, get rid of overcomplicated and unsafe usage in
base_yyerror().

Modified Files:
--------------
pgsql/src/interfaces/ecpg/preproc:
ecpg.header (r1.5 -> r1.6)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/ecpg.header?r1=1.5&r2=1.6)
ecpg.trailer (r1.7 -> r1.8)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/ecpg.trailer?r1=1.7&r2=1.8)
extern.h (r1.71 -> r1.72)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/extern.h?r1=1.71&r2=1.72)