pgsql: Fix unportable usages of tolower().

Log Message:
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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.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        like_match.c (r1.20 -> r1.20.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/like_match.c?r1=1.20&r2=1.20.2.1)
    pgsql/src/interfaces/ecpg/ecpglib:
        execute.c (r1.76 -> r1.76.2.1)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/ecpglib/execute.c?r1=1.76&r2=1.76.2.1)
    pgsql/src/interfaces/ecpg/preproc:
        preproc.y (r1.359.2.1 -> r1.359.2.2)

(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/preproc/preproc.y?r1=1.359.2.1&r2=1.359.2.2)