libtacplus: Allow additional AV pairs to be configured.

* Replace hand-rolled input tokenizer with openpam_readlinev() which supports line continuations and has better quoting and escaping.
* Simplify string handling by merging struct clnt_str and struct srvr_str into just struct tac_str.
* Each server entry in the configuration file can now have up to 255 AV pairs which will be appended to the ones returned by the server in response to a successful authorization request.

This allows nss_tacplus(8) to be used with servers which do not provide identity information beyond confirming the existence of the user.

This adds a dependency on libpam, however libtacplus is currently only used by pam_tacplus(8) (which is already always used with libpam) and the very recently added nss_tacplus(8) (which is extremely niche).  In the longer term it might be a good idea to split this out into a separate library.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	pauamma_gundo.com, markj
Differential Revision:	https://reviews.freebsd.org/D40285
Relnotes:	yes

UPDATING

@@ -27,6 +27,13 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 14.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20230613:
+	Improvements to libtacplus(8) mean that tacplus.conf(5) now
+	follows POSIX shell syntax rules. This may cause TACACS+
+	authentication to fail if the shared secret contains a single
+	quote, double quote, or backslash character which isn't
+	already properly quoted or escaped.
+
 20230612:
 	Belatedly switch the default nvme block device on x86 from nvd to nda.
 	nda created nvd compatibility links by default, so this should be a