Fix issues with writable? detection in brew doctor

Closes Homebrew/homebrew#13689.

Signed-off-by: Max Howell <mxcl@me.com>

There are subtle distinctions between writable? and writable_real? we don't
understand precisely why we need this, but it fixes the bugs :/

Library/Contributions/cmds/brew-unpack.rb

@@ -73,7 +73,7 @@ supplied, patches will also be downloaded and applied.
       unpack_dir.mkpath unless unpack_dir.exist?
     end
 
-    raise "Cannot write to #{unpack_dir}" unless unpack_dir.writable?
+    raise "Cannot write to #{unpack_dir}" unless unpack_dir.writable_real?
 
     formulae.each do |f|
       # Create a nice name for the stage folder.

Library/Contributions/install_homebrew.rb

@@ -84,7 +84,7 @@ chmods = %w( . bin etc include lib lib/pkgconfig Library sbin share var var/log
              share/man/man5 share/man/man6 share/man/man7 share/man/man8
              share/info share/doc share/aclocal ).
             map{ |d| "/usr/local/#{d}" }.
-            select{ |d| File.directory? d and not File.writable? d }
+            select{ |d| File.directory? d and not File.writable_real? d }
 chgrps = chmods.reject{ |d| File.stat(d).grpowned? }
 
 unless chmods.empty?

Library/Homebrew/cmd/doctor.rb

@@ -279,7 +279,7 @@ def __check_subdir_access base
 
   target.find do |d|
     next unless d.directory?
-    cant_read << d unless d.writable?
+    cant_read << d unless d.writable_real?
   end
 
   cant_read.sort!
@@ -300,7 +300,7 @@ end
 def check_access_usr_local
   return unless HOMEBREW_PREFIX.to_s == '/usr/local'
 
-  unless Pathname('/usr/local').writable? then <<-EOS.undent
+  unless Pathname('/usr/local').writable_real? then <<-EOS.undent
     The /usr/local directory is not writable.
     Even if this directory was writable when you installed Homebrew, other
     software may change permissions on this directory. Some versions of the
@@ -322,7 +322,7 @@ end
 
 def __check_folder_access base, msg
   folder = HOMEBREW_PREFIX+base
-  if folder.exist? and not folder.writable?
+  if folder.exist? and not folder.writable_real?
     <<-EOS.undent
       #{folder} isn't writable.
       This can happen if you "sudo make install" software that isn't managed

Library/Homebrew/cmd/install.rb

@@ -33,8 +33,8 @@ module Homebrew extend self
   end
 
   def check_writable_install_location
-    raise "Cannot write to #{HOMEBREW_CELLAR}" if HOMEBREW_CELLAR.exist? and not HOMEBREW_CELLAR.writable?
-    raise "Cannot write to #{HOMEBREW_PREFIX}" unless HOMEBREW_PREFIX.writable? or HOMEBREW_PREFIX.to_s == '/usr/local'
+    raise "Cannot write to #{HOMEBREW_CELLAR}" if HOMEBREW_CELLAR.exist? and not HOMEBREW_CELLAR.writable_real?
+    raise "Cannot write to #{HOMEBREW_PREFIX}" unless HOMEBREW_PREFIX.writable_real? or HOMEBREW_PREFIX.to_s == '/usr/local'
   end
 
   def check_xcode

Library/Homebrew/extend/pathname.rb

@@ -353,7 +353,7 @@ class Pathname
             To list all files that would be deleted:
               brew link -n formula_name
             EOS
-        elsif !dirname.writable?
+        elsif !dirname.writable_real?
           raise <<-EOS.undent
             Could not symlink file: #{src.expand_path}
             #{dirname} is not writable. You should change its permissions.
@@ -375,7 +375,7 @@ class Pathname
 
   def ensure_writable
     saved_perms = nil
-    unless writable?
+    unless writable_real?
       saved_perms = stat.mode
       chmod 0644
     end

Library/Homebrew/global.rb

@@ -16,7 +16,7 @@ def cache
     # we do this for historic reasons, however the cache *should* be the same
     # directory whichever user is used and whatever instance of brew is executed
     home_cache = Pathname.new("~/Library/Caches/Homebrew").expand_path
-    if home_cache.directory? and home_cache.writable?
+    if home_cache.directory? and home_cache.writable_real?
       home_cache
     else
       root_cache = Pathname.new("/Library/Caches/Homebrew")