Document the wild hostname(1) issue on OS X

As described in #82, the hostname on OS X can vary:

- The result of hostname(1) can change depending on DHCP settings; by
  default, as you move between networks, your hostname will change.
- The value for `ComputerName` as set in scutil(8) contains values that
  are inappropriate for a directory name (spaces, quotes, and so on),
  and is blank by default.
- The value for `LocalHostName` changes based on DNS and DHCP settings.

Therefore, OS X users are highly encouraged to set a hostname using the
`HOSTNAME` variable in rcrc(5).
Update documentation on usage

New flags have accumulated without proper care for the usage
instructions or man pages.

I manually went through each program and verified its usage instruction
against its `getopts`, then I alphabeticalized the usage message.

Based on the usage message, I then verified the synposis in the manpage.
Then, based on the synposis, I alphabeticalized the detailed listing of
the arguments and filled in the missing pieces. The `-h` and `-V`
arguments were missing from all manpages.

In the future we will need to be more careful about this. It would be
good to automate a checker that refuses to build unless the docs have
all the flags mentioned.