Change `brew linkapps` to use the global /Applications folder

*(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].)