sort: correctly inherit global flags for keys (#2302)

Closes #2254. We should only inherit global settings for keys when there
are absolutely no options attached to the key.

The default key (matching the whole line) is implicitly added only if no
keys are supplied.

Improved some error messages by including more context.

tests/fixtures/sort/blanks.expected

@@ -0,0 +1,5 @@
+  a
+b
+         x
+  x
+ z

tests/fixtures/sort/blanks.expected.debug

@@ -0,0 +1,15 @@
+  a
+  _
+___
+b
+_
+_
+         x
+         _
+__________
+  x
+  _
+___
+ z
+ _
+__

tests/fixtures/sort/blanks.txt

@@ -0,0 +1,5 @@
+b
+  a
+ z
+  x
+         x

tests/fixtures/sort/keys_ignore_flag.expected

@@ -0,0 +1,2 @@
+ 1a
+1A

tests/fixtures/sort/keys_ignore_flag.expected.debug

@@ -0,0 +1,6 @@
+ 1a
+ _
+___
+1A
+_
+__

tests/fixtures/sort/keys_ignore_flag.txt

@@ -0,0 +1,2 @@
+ 1a
+1A