setclasspriority(): New possible value 'inherit'

It indicates to the login.conf machinery (setusercontext() /
setclasscontext()) to leave priority alone, effectively inheriting it
from the parent process.

PR:                     271749
Reviewed by:            emaste, yuripv
Approved by:            emaste (mentor)
MFC after:              3 days
Relnotes:               yes
Sponsored by:           Kumacom SAS
Differential Revision:  https://reviews.freebsd.org/D40690

lib/libutil/login_class.c

@@ -462,9 +462,15 @@ static void
 setclasspriority(login_cap_t * const lc, struct passwd const * const pwd)
 {
 	const rlim_t def_val = LOGIN_DEFPRI, err_val = INT64_MIN;
-	rlim_t p = login_getcapnum(lc, "priority", def_val, err_val);
+	rlim_t p;
 	int rc;
 
+	/* If value is "inherit", nothing to change. */
+	if (login_getcapenum(lc, "priority", inherit_enum) == 0)
+		return;
+
+	p = login_getcapnum(lc, "priority", def_val, err_val);
+
 	if (p == err_val) {
 		/* Invariant: 'lc' != NULL. */
 		syslog(LOG_WARNING,