su: move to bmake and remove some features

su: move to bmake and remove some features

 - embedded_su is removed. there are no consumers in-tree.
 - su.static is removed. /usr is always mounted (and by default it's in
   the same fs as /), no need to pretend it isn't.
 - remove /etc/default/su. read PATH, SUPATH and SLEEPTIME from
   /etc/default/login instead.
 - remove CONSOLE, SULOG and SYSLOG. always log to syslog; syslog
   configuration can be used to redirect output to the console or a
   different log file if desired.
 - stop setting HZ and MAIL in environment
 - compile with -Wall -Werror