pgsql: Fix unportable usages of tolower().

Log Message:
-----------
Fix unportable usages of tolower(). On signed-char machines, it is necessary
to explicitly cast the output back to char before comparing it to a char
value, else we get the wrong result for high-bit-set characters. Found by
Rolf Jentsch. Also, fix several places where <ctype.h> functions were being
called without casting the argument to unsigned char; this is likewise
unportable, but we keep making that mistake :-(. These found by buildfarm
member salamander, which I will desperately miss if it ever goes belly-up.

Modified Files:
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pgsql/src/backend/utils/adt:
like_match.c (r1.20 -> r1.21)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/like_match.c?r1=1.20&r2=1.21)
pgsql/src/interfaces/ecpg/ecpglib:
execute.c (r1.76 -> r1.77)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/ecpglib/execute.c?r1=1.76&r2=1.77)
pgsql/src/interfaces/ecpg/preproc:
preproc.y (r1.361 -> r1.362)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/preproc.y?r1=1.361&r2=1.362)