pgsql: Add resetStringInfo(), which clears the content of a StringInfo,

Log Message:
-----------
Add resetStringInfo(), which clears the content of a StringInfo, and
fixup various places in the tree that were clearing a StringInfo by hand.
Making this function a part of the API simplifies client code slightly,
and avoids needlessly peeking inside the StringInfo interface.

Modified Files:
--------------
    pgsql/contrib/tablefunc:
        tablefunc.c (r1.46 -> r1.47)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/tablefunc/tablefunc.c.diff?r1=1.46&r2=1.47)
    pgsql/src/backend/catalog:
        pg_shdepend.c (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_shdepend.c.diff?r1=1.16&r2=1.17)
    pgsql/src/backend/commands:
        copy.c (r1.276 -> r1.277)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/copy.c.diff?r1=1.276&r2=1.277)
    pgsql/src/backend/lib:
        stringinfo.c (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/lib/stringinfo.c.diff?r1=1.44&r2=1.45)
    pgsql/src/backend/libpq:
        pqcomm.c (r1.190 -> r1.191)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/pqcomm.c.diff?r1=1.190&r2=1.191)
    pgsql/src/backend/storage/lmgr:
        deadlock.c (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/deadlock.c.diff?r1=1.45&r2=1.46)
    pgsql/src/backend/tcop:
        fastpath.c (r1.95 -> r1.96)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/fastpath.c.diff?r1=1.95&r2=1.96)
        postgres.c (r1.526 -> r1.527)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.526&r2=1.527)
    pgsql/src/backend/utils/adt:
        rowtypes.c (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/rowtypes.c.diff?r1=1.18&r2=1.19)
        xml.c (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.33&r2=1.34)
    pgsql/src/include/lib:
        stringinfo.h (r1.33 -> r1.34)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/lib/stringinfo.h.diff?r1=1.33&r2=1.34)