check for preexisting PAM credentials in .irodsA before overwriting

[d-w-moore]

In issue #621, the free function write_pam_credentials_to_secrets_file is created to provide iinit-like capability for writing a .irodsA containing PAM credentials. However, unlike iinit, this function's use might be insensitive to, and destroy, a pre-existing .irodsA file.

So, we need to examine this. At the very least, we could add an option to the function, specifying whether an already existing .irodsA should be checked for , prior to negotiating a new PAM password and overwriting the existing .irodsA.