This makes sure the return code of extracting the path from manpath(1)
is not ignored. Also, unset the MANPATH environment variable instead of
just assigning an empty string to it, so that manpath(1) determines a
suitable default hierarchy to search for manual pages.
The way I was doing the test is incorrect, as [ is the test(1) program
and the shell builtin command is not something it understands. Instead,
I now just let the shell execute `command` and then set the variable if
`brew` is available.
