Set $MANPATH (#1005)

Currently, man has issues finding man pages for Nix-installed
application (also, `nix-env --help` doesn't work). The issue is caused
by custom `$MANPATH` set by my system. That makes man use it instead of
searching in default location.

Either of next lines workaround the issue:
```sh
unset MANPATH

export MANPATH=$HOME/.nix-profile/share/man:$MANPATH
```

This patch adds the later line to the `nix-profile.sh` if user has
`MANPATH` set. (Not clearing `MANPATH` as that would be disrespect of
user's preferences.)

As a side-effect, host's man might find man pages installed by Nix.

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