Add man proxy to rustup for viewing rust-toolchain manpages

[tesuji]

Describe the problem you are trying to solve

I cannot directly use man cargo, man rustc, etc.

Describe the solution you'd like

The new command might be: rustup man (which is somewhat similar to rustup completions).

As @kinnison suggest, we could use the man proxy of rustup. This command could alter
the MANPATH to point to appropriate rust-toolchain's manpages and open them with
system man program.

Some examples:

Command	Meaning
rustup man cargo	Display manpages of cargo
rustup man cargo-new	Display manpages of cargo new
rustup +nightly man rustc	Display manpages of nightly rustc
rustup man install	Symlink-install manpages to ~/.local/share/man/man1

[kinnison]

I fear we'd have to add a man proxy to rustup which altered the environment to include an appropriate MANPATH and then ran the system version of man. I don't see how any other approach would work given we have to find which manpages to display based on rust-toolchain and friends.

[tesuji]

Closed in #616.