MFC 271381-271382,271385,271463-271466,271485,271506

o Add sysctls to enable/disable potentially dengerous key combinations, like

reboot/halt/debug.

o Add support for most key combinations supported by syscons(4).
o Some spelling fixes
o Remove stray whitespaces.
o Switch vt(4) to traditional behaviour with copy-paste same as syscons(4) do.
o Fix stray char on paste.
o Fix 'function declaration isn't a prototype' warning.
o vt(4): Enclose vt_mouse_paste() prototype inside #ifndef SC_NO_CUTPASTE/#endif

Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation