reboot: Default to a clean shutdown

• If invoked as fasthalt or fastboot, behavior is unchanged.

• If not invoked as fasthalt or fastboot, we simply signal init(8), just like shutdown(8) does, instead of taking the system down ourselves.

• Since only init can handle the RB_REROOT case, the -r flag is not supported in fast mode.

• Update the usage string to correctly reflect the program being run (fast or normal; halt, boot, or nextboot) and the options available in each case.

• Update the manual page to make the distinction between normal and fast mode clear, better explain what shutdown(8) still does that reboot(8) does not, and add a historical note explaining what the difference between the two used to be.

MFC after: 1 month
Relnotes: yes
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D54117