Migrate PATH-completion logic from complete.cpp to expand.cpp

Prior to this fix, when completing a command that doesn't have a /, we
would prepend each component of PATH and then expand the whole thing. So
any special characters in the PATH would be interpreted when performing
tab completion.

With this fix, we pull the PATH resolution out of complete.cpp and
migrate it to expand.cpp. This unifies nicely with the CDPATH resolution
already in that file. This requires introducing a new expand flag
EXPAND_SPECIAL_FOR_COMMAND, which is analogous to EXPAND_SPECIAL_CD
(which is renamed to EXPAND_SPECIAL_FOR_CD). This flag tells expand to
resolve paths against PATH instead of the working directory.

Fixes #952

tests/test6.in

@@ -72,3 +72,37 @@ if begin; rm -rf test6.tmp.dir; and mkdir test6.tmp.dir; end
 else
     echo "error: could not create temp environment" >&2
 end
+
+# Test command expansion with parened PATHs (#952)
+begin
+  set -l parened_path $PWD/'test6.tmp2.(paren).dir'
+  set -l parened_subpath $parened_path/subdir
+  if not begin
+        rm -rf $parened_path
+	    and mkdir $parened_path
+		and mkdir $parened_subpath
+	    and ln -s /bin/ls $parened_path/'__test6_(paren)_command'
+	    and ln -s /bin/ls $parened_subpath/'__test6_subdir_(paren)_command'
+      end
+    echo "error: could not create command expansion temp environment" >&2
+  end
+  
+  # Verify that we can expand commands when PATH has parens
+  set -l PATH $parened_path $PATH
+  set -l completed (complete -C__test6_ | cut -f 1 -d \t)
+  if test "$completed" = '__test6_(paren)_command'
+    echo "Command completion with parened PATHs test passed"
+  else
+    echo "Command completion with parened PATHs test failed. Expected __test6_(paren)_command, got $completed" >&2
+  end
+  
+  # Verify that commands with intermediate slashes do NOT expand with respect to PATH
+  set -l completed (complete -Csubdir/__test6_subdir)
+  if test -z "$completed"
+    echo "Command completion with intermediate slashes passed"
+  else
+    echo "Command completion with intermediate slashes: should output nothing, instead got $completed" >&2
+  end
+  
+  rm -rf $parened_path
+end

tests/test6.out

@@ -33,3 +33,5 @@ CCCC:
 implicit cd complete works
 no implicit cd complete after 'command'
 PATH does not cause incorrect implicit cd
+Command completion with parened PATHs test passed
+Command completion with intermediate slashes passed