Fix man proxy in FreeBSD

[tesuji]

In FreeBSD, man command only searches for man files in MANPATH.
Which means that FreeBSD man program does not accept interpret local man files.

On Bash-compatible shell, one could use this command to open rust specific manpages:

env MANPATH=$HOME/.rustup/toolchains/stable-x86_64-unknown-freebsd/share/man man rustc

Locally tested on:

• Ubuntu
• FreeBSD 11

r? @kinnison

Note: Ping me if someone needs to squash this PR.

[kinnison]

I think this looks good, though I'm loathe to approve because -M overrides the MANPATH environment variable which might also be set and relevant. I'd be more comfortable with manipulating that than passing -M

[tesuji]

@kinnison So you want to manipulate MANPATH env var via Command::env?

[kinnison]

I figure that if MANPATH is set, we have to do that to be correct (i.e. prepend our path onto MANPATH) If it's not set, we could use -M but at that point the code is simpler to always prepend to MANPATH

[tesuji]

@kinnison I pushed the change you requested.

[kinnison]

This looks reasonable to me. Have you verified it against FreeBSD and at least one Linux?

[kinnison]

If so, please rebase to clean up the history of the branch before I merge.

[tesuji]

@kinnison I tested on Ubuntu. FreeBSD I only test the equivalent shell command.

[kinnison]

LGTM