Change brew linkapps to use the global /Applications folder

[ELLIOTTCABLE]

(As per @adamv's request, re-requesting this pull, here. I'll keep this rebased as necessary as the discussion continues.)

This has been heavily discussed elsewhere. I'll simply re-list the reasons for this change, here:

• Less surprising to everyday users
• Unlike other ‘global’ directories, won't munge up the system, as it's a ‘global’ directory that everyday users generally modify

In addition, of some relevance, it seems that ~/Applications doesn't even exist by default in Mavericks, which suggests to me that Apple has realized that very few people have any use for it, and has concluded that it does very little except cause user-confusion amongst those who don't know how/why to create it. That seems in-line with the argument I've been making.

Also, it seems to be the majority request, here. At an informal reading of #8699 and #17643 (and please, forgive me, or let me know, if I mis-read your intentions, posters), the following are for the global /Applications being the default, with the user-~/Applications being an optional flag: @hced, @dysinger, @zhouyan, @mxcl, @thecontrarian, @iainbeeston, @demure, and @vilhalmer. As far as I can tell, the only person categorically against it, is @adamv. )'=

Other relevant Issues / Pull Requests: #12619, #18196.

Further commentary welcome. Also, if any of my changes to the code are unacceptable, please let me know; CONTRIBUTING.md was rather unhelpful to someone modifying Homebrew itself, ironically. (=

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(I'll be forthcoming with additional patches, not dependent on this one, that respectively add documentation of this feature, since there seems to be none [#22380]; and another controversial one wherein I suggest that we move apps, not symlink them, into whichever folder we decide upon here [#22379].)

[adamv]

For a local link, this existence check still needs to be done.

[ELLIOTTCABLE]

I'll fix that up and push a new commit.

[ELLIOTTCABLE]

I'm going to push the commit(s) from #22380 here, and leave some time for them to be reviewed in-full; then I'll squash and rebase on top of the latest master after a sign-off from one of you maintainers, so it's a one-click merge.

[adamv]

@BrewTestBot test this please

[adamv]

Looks fine; go ahead and add another commit that moves this from contributions to the main cmd folder too.

[ELLIOTTCABLE]

@adamv is there any other changes necessary in the codebase to support that? I'm not familiar with how these are loaded. Is it entirely dynamic (i.e. just a git mv)?

I've pushed that change, let me know if I missed something.

Edit: Yeah, there's clearly more required. Maybe it's better than somebody else make this change, as I'm fairly swamped (unless it's something simple that I'm missing here):

> brew linkapps
Linking /usr/local/Cellar/macvim/7.4-71/MacVim.app
Error: /Applications/MacVim.app already exists, skipping.
Linking /usr/local/Cellar/python/2.7.5/Build Applet.app
Linking /usr/local/Cellar/python/2.7.5/IDLE.app
Linking /usr/local/Cellar/python/2.7.5/Python Launcher.app
Finished linking. Find the links under /Applications.
Error: undefined method `linkapps' for Homebrew:Module
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Library/brew.rb:95:in `send'
/usr/local/Library/brew.rb:95

[adamv]

Ah, right, there's a difference in form. If you squash the original 4 commits into 1 or 2 commits, I can pull those then move it myself.

Otherwise, look at the files in Homebrew/cmd for the method naming convention.

[ELLIOTTCABLE]

Okay, @adamv. I've rebased everything on top of the current master, and then squashed the relevant commits into a single one for you to cherry-pick:

ELLIOTTCABLE/homebrew@a4273ae