login.conf(5): split MAIL env var out into a "mail" capability

This allows it to be easily suppressed in, e.g., the "daemon" class where it
will not be properly expanded.

This is a part of D21481.

Submitted by:	Andrew Gierth <andrew_tao173.riddles.org.uk>

lib/libutil/login.conf.5

@@ -19,7 +19,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd July 8, 2011
+.Dd January 19, 2020
 .Dt LOGIN.CONF 5
 .Os
 .Sh NAME
@@ -237,6 +237,7 @@ for details.
 .It "label	string		Default MAC policy; see"
 .Xr maclabel 7 .
 .It "lang	string		Set $LANG environment variable to the specified value."
+.It "mail	string		Set $MAIL environment variable to the specified value."
 .It "manpath	path		Default search path for manpages."
 .It "nocheckmail	bool	false	Display mail status at login."
 .It "nologin	file		If the file exists it will be displayed and"

lib/libutil/login_class.c

@@ -131,6 +131,7 @@ static struct login_vars {
 }, envars[] = {
     { "lang",           "LANG",       NULL, 1},
     { "charset",        "MM_CHARSET", NULL, 1},
+    { "mail",           "MAIL",       NULL, 1},
     { "timezone",       "TZ",         NULL, 1},
     { "term",           "TERM",       NULL, 0},
     { NULL,             NULL,         NULL, 0}

usr.bin/login/login.conf

@@ -26,7 +26,8 @@ default:\
 	:passwd_format=sha512:\
 	:copyright=/etc/COPYRIGHT:\
 	:welcome=/var/run/motd:\
-	:setenv=MAIL=/var/mail/$,BLOCKSIZE=K:\
+	:setenv=BLOCKSIZE=K:\
+	:mail=/var/mail/$:\
 	:path=/sbin /bin /usr/sbin /usr/bin /usr/local/sbin /usr/local/bin ~/bin:\
 	:nologin=/var/run/nologin:\
 	:cputime=unlimited:\
@@ -61,6 +62,7 @@ xuser:\
 staff:\
 	:tc=default:
 daemon:\
+	:mail@:\
 	:memorylocked=128M:\
 	:tc=default:
 news:\