rcorder(8): add support for /etc/rc.resume, so it calls "rcorder -k resume"
and runs scripts containing "KEYWORD: resume" with single "resume" argument.

Working example is the port sysutils/cpupdate that defines
extra_commands="resume" to reload CPU microcode cleared
by suspend/resume sequence.

This change does nothing for a system having no scripts with KEYWORD: resume.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D15247