Fix memory leakage related to tac_svr

[stanAtAtl]

The memory blocks pointed by tac_srv[] are dynamically allocated in _pam_parse() when
server address in option "server" is parsed. But each time _pam_parse() is called,
tac_srv[] is nulled without de-allocating, so those blocks of memroy get "lost".

Those blocks of memory are allocated by getaddrinfo(), this patch uses freeaddrinfo()
to free them. Before releasing the memory, the actual address and key information is
saved to static memory.

Since act_srv[] may have different ip addresses and keys each time _pam_parse() is called,
the pointers in active_server may point to different server rather than the server being
active last time. This patch saves the active server address and key to specific static
memory and let active_server point to them.

[pprindeville]

@stanAtAtl, @jeroennijhof: I'm looking at this PR, and I'm thinking that it's partially at odds with PR #72 and PR #78, which attempt to move server parameters (like the secret) into a tac_session structure instead.

Maybe we should coordinate changes?