remove libwrap support

libwrap is an obscure artefact from a past long ago, when source IP
address meant something.

And its API is "interesting"; it requires the application to expose
two global variables `allow_severity` and `deny_severity`.  This led
to bug #437.  I don't want to declare those variables; instead, I'd
like to remove libwrap support.

Closes #437

src/lib/wrap/meson.build

@@ -1,21 +0,0 @@
-libwrap_option = get_option('libwrap')
-enable_libwrap = false
-if not libwrap_option.disabled() and compiler.has_header('tcpd.h') and compiler.compiles('''
-#include <tcpd.h>
-bool CheckLibWrap(int fd, const char &progname) {
-	struct request_info req;
-	request_init(&req, RQ_FILE, fd, RQ_DAEMON, progname, 0);
-	fromhost(&req);
-	return hosts_access(&req);
-}
-''')
-  libwrap_dep = compiler.find_library('wrap', required: libwrap_option)
-else
-  libwrap_dep = dependency('', required: libwrap_option)
-endif
-
-if not libwrap_dep.found() and libwrap_option.enabled()
-  error('libwrap not found')
-endif
-  
-conf.set('HAVE_LIBWRAP', libwrap_dep.found())