pgexternaltable - src: fix the pg_try and pg_catch memory context

Log Message:
-----------
fix the pg_try and pg_catch memory context recover problem and add the base class Buffer for further expanding

Modified Files:
--------------
    src/externaltable:
        FileBuffer.cc (r1.1 -> r1.2)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/FileBuffer.cc.diff?r1=1.1&r2=1.2)
        FileBuffer.h (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/FileBuffer.h.diff?r1=1.1&r2=1.2)
        Interface_FileBuffer.cc (r1.1 -> r1.2)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/Interface_FileBuffer.cc.diff?r1=1.1&r2=1.2)
        Makefile (r1.1 -> r1.2)
        (http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/Makefile.diff?r1=1.1&r2=1.2)
        externaltab.c (r1.1 -> r1.2)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/externaltab.c.diff?r1=1.1&r2=1.2)
        getExtTabInfo.c (r1.1 -> r1.2)

(http://cvs.pgfoundry.org/cgi-bin/cvsweb.cgi/pgexternaltable/src/externaltable/getExtTabInfo.c.diff?r1=1.1&r2=1.2)