cargo: Add local mode and use it by default

[tychosci]

Issue #1756

The changes are the following:

• Add local mode which does operate with project's ".cargo" rather than "~/.cargo".
• Use local mode by default. you can change it with "-g" or "--global", for global mode operation.
• Add functions that required to add lib directory in project's ".cargo" to linker search path.
• Use std::getopts in cargo.rs to avoid arguments ordering problems.

[graydon]

Hm. Can we rephrase this as 3 modes: local, user and system? "Global" is a bit ambiguous (user/system) and we have a sysroot that the compiler at least knows about, can consider viable for system-level installation.

[graydon]

Otherwise cool, btw :)

[tychosci]

Can we rephrase this as 3 modes: local, user and system?

I think that's better!
I'll add commit to get it to work, it seems to be not so difficult :)

[tychosci]

changes from previous commit:

• 2 modes (local/global) to 3 modes (system/user/local)
• Add note about modes to usage
• Show the information about where .cargo places when .cargo is successfully initialized
• Add function that required to grab path of system-level .cargo from cargo.rs

[brson]

Hi tychosci. This is looking great, but I have one quibble about how the system cargo files are installed. This appears to put them under $sysroot/.cargo/... which in practice is /usr/local/.cargo. This is a strange place for us to be dropping a custom folder. I would expect the system install to not use the .cargo folder and just put libs under /usr/local/lib, bins under /usr/local/bin so that they are on all the normal search paths (rustc already looks here). I can make this change myself, but I'd like to hear your and @graydon's opinion.

[brson]

Oh, if we did that though we would not have a place to put all of cargo's work files.

[brson]

OK. I pushed this but disabled system mode so we can reconsider how it works. Added #1795. Thanks!

[tychosci]

Ah, I did not notice about that, since I always install rustc and its libs and tools with --prefix :(

I agree that /usr/local is weird place to put .cargo directory for that reason, and it's really good to put installed bin/libs into /usr/local/bin or /usr/local/lib, because we don't need to add another path to $PATH if it's so.

I personally think /usr/local/lib/cargo is good place to store system-level resources for cargo, and symlinking bin/libs in /usr/local/lib/cargo/bin and /usr/local/lib/cargo/lib to /usr/local/bin/<bin> and /usr/local/lib/<lib> after the installation with cargo install <package> is really great.

Furthermore, I think all operations by user-mode and local-mode should use resources of system-mode's cargo.
I realized it's annoying thing that "whenever we create new project and install libraries with cargo, it's necessary to execute cargo init and cargo sync at first", while I was testing cargo with N-modes.