pgsql: Add port support for unsetenv() in back branches.

Log Message:
-----------
Add port support for unsetenv() in back branches.  Needed for locale
environment fix.

Tags:
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REL7_4_STABLE

Modified Files:
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    pgsql:
        configure (r1.310.2.18 -> r1.310.2.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.diff?r1=1.310.2.18&r2=1.310.2.19)
        configure.in (r1.301.2.16 -> r1.301.2.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in.diff?r1=1.301.2.16&r2=1.301.2.17)
    pgsql/src/include:
        pg_config.h.in (r1.64.2.3 -> r1.64.2.4)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config.h.in.diff?r1=1.64.2.3&r2=1.64.2.4)
        port.h (r1.14 -> r1.14.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/port.h.diff?r1=1.14&r2=1.14.2.1)

Added Files:
-----------
    pgsql/src/port:
        unsetenv.c (r1.6.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/port/unsetenv.c?rev=1.6.2.1&content-type=text/x-cvsweb-markup)