- Add a global option where we can protect processes when swap space

- Add a global option where we can protect processes when swap space
  is exhausted.

How to use:

Basically we need to add on rc.conf an another option like:

    If we want to protect only the main processes.
    syslogd_oomprotect="YES"

    If we want to protect all future children of the specified processes.
    syslogd_oomprotect="ALL"

PR:		204741 (based on)
Submitted by:	eugen@grosbein.net
Reviewed by:	jhb, allanjude, rpokala and bapt
MFC after:	4 weeks
Relnotes:	Yes
Sponsored by:	gandi.net
Differential Revision:	https://reviews.freebsd.org/D5176