pgsql-server: Fix linking problem when enabling thread safety on Darwin:

Log Message:
-----------
Fix linking problem when enabling thread safety on Darwin: uninitialized
global variables are problematic on this platform.  Simplest solution
seems to be to initialize pthread key variable to 0.  Also, rename this
variable and check_sigpipe_handler to something involving "pq" to
avoid gratuitous pollution of application namespace.

Modified Files:
--------------
    pgsql-server/src/interfaces/libpq:
        fe-connect.c (r1.279 -> r1.280)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/interfaces/libpq/fe-connect.c.diff?r1=1.279&r2=1.280)
        fe-print.c (r1.51 -> r1.52)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/interfaces/libpq/fe-print.c.diff?r1=1.51&r2=1.52)
        fe-secure.c (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/interfaces/libpq/fe-secure.c.diff?r1=1.45&r2=1.46)
        libpq-int.h (r1.88 -> r1.89)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/interfaces/libpq/libpq-int.h.diff?r1=1.88&r2=1.89)