Optional 'init' argument to 'string.gmatch'

The function 'string.gmatch' now has an optional 'init' argument,
similar to 'string.find' and 'string.match'. Moreover, there was
some reorganization in the manipulation of indices in the string
library.
This commit also includes small janitorial work in the manual
and in comments in the interpreter loop.

testes/pm.lua

@@ -297,6 +297,35 @@ for k,v in pairs(t) do assert(k+1 == v+0); a=a+1 end
 assert(a == 3)
 
 
+do   -- init parameter in gmatch
+  local s = 0
+  for k in string.gmatch("10 20 30", "%d+", 3) do
+    s = s + tonumber(k)
+  end
+  assert(s == 50)
+
+  s = 0
+  for k in string.gmatch("11 21 31", "%d+", -4) do
+    s = s + tonumber(k)
+  end
+  assert(s == 32)
+
+  -- there is an empty string at the end of the subject
+  s = 0
+  for k in string.gmatch("11 21 31", "%w*", 9) do
+    s = s + 1
+  end
+  assert(s == 1)
+
+  -- there are no empty strings after the end of the subject
+  s = 0
+  for k in string.gmatch("11 21 31", "%w*", 10) do
+    s = s + 1
+  end
+  assert(s == 0)
+end
+
+
 -- tests for `%f' (`frontiers')
 
 assert(string.gsub("aaa aa a aaa a", "%f[%w]a", "x") == "xaa xa x xaa x")