Use library constructor/destructor to initialize OpenSSL

Instead of calling isc_tls_initialize()/isc_tls_destroy() explicitly use
gcc/clang attributes on POSIX and DLLMain on Windows to initialize and
shutdown OpenSSL library.

This resolves the issue when isc_nm_create() / isc_nm_destroy() was
called multiple times and it would call OpenSSL library destructors from
isc_nm_destroy().

At the same time, since we now have introduced the ctor/dtor for libisc,
this commit moves the isc_mem API initialization (the list of the
contexts) and changes the isc_mem_checkdestroyed() to schedule the
checking of memory context on library unload instead of executing the
code immediately.

lib/dns/openssl_link.c

@@ -68,8 +68,6 @@ isc_result_t
 dst__openssl_init(const char *engine) {
 	isc_result_t result = ISC_R_SUCCESS;
 
-	isc_tls_initialize();
-
 	enable_fips_mode();
 
 #if !defined(OPENSSL_NO_ENGINE)
@@ -110,7 +108,6 @@ dst__openssl_destroy(void) {
 	}
 	e = NULL;
 #endif /* if !defined(OPENSSL_NO_ENGINE) */
-	isc_tls_destroy();
 }
 
 static isc_result_t