pam_unix: Support reading YESCRYPT_COST_FACTOR from /etc/login.defs

[dvzrv]

Hi! I package PAM integration and shadow for Arch Linux.

We are currently discussing the move to yescrypt and want to rely on PAM.
As integration with shadow's /etc/login.defs will remain though, we would love to see the possibility to also retrieve rounds from YESCRYPT_COST_FACTOR in /etc/login.defs.

Currently it appears that the rounds= parameter is supported for yescrypt:

linux-pam/modules/pam_unix/support.c

Lines 188 to 195 in 77bd338

	/* Enforce sane "rounds" values */
	if (on(UNIX_ALGO_ROUNDS, ctrl)) {
	if (on(UNIX_GOST_YESCRYPT_PASS, ctrl) ||
	on(UNIX_YESCRYPT_PASS, ctrl)) {
	if (*rounds < 3)
	*rounds = 3;
	else if (*rounds > 11)
	*rounds = 11;

Seeing how an integration with /etc/login.defs is supported for SHA256 and SHA512

linux-pam/modules/pam_unix/support.c

Lines 101 to 111 in 77bd338

	/* read number of rounds for crypt algo */
	if (rounds && (on(UNIX_SHA256_PASS, ctrl) || on(UNIX_SHA512_PASS, ctrl))) {
	val = pam_modutil_search_key(pamh, LOGIN_DEFS, "SHA_CRYPT_MAX_ROUNDS");
	if (val) {
	*rounds = strtol(val, NULL, 10);
	set(UNIX_ALGO_ROUNDS, ctrl);
	free (val);
	}
	}
	}

it would be amazing to also be able to use the same kind of integration for yescrypt!