Implement new read --null flag

The `--null` flag to `read` makes it split incoming lines on NUL instead
of newlines. This is intended for processing the output of a command
that uses NUL separators (such as `find -print0`).

Fixes #1694.

tests/read.in

@@ -1,3 +1,4 @@
+# vim: set filetype=fish:
 #
 # Test read builtin and IFS
 #
@@ -35,6 +36,8 @@ echo '' | read -l one two
 print_vars one two
 echo 'test' | read -l one two three
 print_vars one two three
+echo 'foo   bar   baz' | read -l one two three
+print_vars one two three
 
 echo
 set -l IFS
@@ -91,3 +94,25 @@ echo $foo
 echo $bar
 echo 'test' | read -n 1 foo
 echo $foo
+
+# read -0 tests
+
+echo
+echo '# read -z tests'
+echo -n 'testing' | read -lz foo
+echo $foo
+echo -n 'test ing' | read -lz foo
+echo $foo
+echo 'newline' | read -lz foo
+echo $foo
+echo -n 'test ing' | read -lz foo bar
+print_vars foo bar
+echo -ne 'test\0ing' | read -lz foo bar
+print_vars foo bar
+echo -ne 'foo\nbar' | read -lz foo bar
+print_vars foo bar
+echo -ne 'foo\nbar\0baz\nquux' | while read -lza foo
+    print_vars foo
+end
+
+true

tests/read.out

@@ -16,6 +16,7 @@ two
 1 ''
 1 '' 1 ''
 1 'test' 1 '' 1 ''
+1 'foo' 1 'bar' 1 '  baz'
 
 1 'hello'
 1 'h' 1 'ello'
@@ -42,3 +43,14 @@ test
 tes
 tin
 t
+
+# read -z tests
+testing
+test ing
+newline
+
+1 'test' 1 'ing'
+1 'test' 1 ''
+1 'foo' 1 'bar'
+2 'foo' 'bar'
+2 'baz' 'quux'