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

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.

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. (=

----

(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; and another controversial one wherein I suggest that we *move* apps, not symlink them, into whichever folder we decide upon here.)