Allow overriding rctl.conf(5) file location for /etc/rc.d/rctl

Reviewed by:	trasz
Approved by:	bapt (mentor)
MFC after:      1 week
Add forgotten rc.conf(5) update with r261931

Reviewed by:	trasz
Approved by:	bapt (mentor)
MFC after:	1 week
Rework rctl(8) manpage

- Note that kernel options are required
- Shift parameters around in SYNOPSIS to make it more clear that there are
  different modes
- For all literal symbols such as 'process' or 'loginclass' or 'wallclock',
  etc, make them into bold symbols with .Sy
- For each subject:subject-id:etc: use .Em to underline to make it more clear
  they relate to the rule syntax
- Document how devd(8) support works
- Move RSS warning to BUGS and replace RSS with 'memoryuse' since 'RSS' is not
  defined in the manpage
- Add more examples around listing existing rules
- Make rule syntax into a list to improve readability
- Add a list of subjects and their corresponding subject-id same as
  RESOURCES/ACTIONS have lists
- Note that rctl(8) takes affect on all current and future processes
- Note that amount can take human numbers
- Add reference to login.conf(5) in few places login class is mentioned

Reviewed by:	trasz
Approved by:	bapt (mentor)
MFC after:	1 week