Bug 4234: comm_connect_addr uses errno incorrectly

Bug 4234: comm_connect_addr uses errno incorrectly

comm_connect_addr() uses errno to determine whether library calls like connect()
are successful. Its callers also use errno for extra information on the cause
of any problem. However, after calling library functions like connect(),
comm_connect_addr() calls other library functions which can overwrite errno.

As the errno manpage explains, "a function that succeeds is allowed to change
errno". So even when nothing is wrong, comm_connect_addr() may return an error
flag if libc sets errno. And when something *is* wrong, incorrect error information
may be returned to the caller because errno was overwritten with a different code.

Correct this by using our own error code variable which is set only when a library
call fails. To avoid breaking callers, set errno before returning.