fio: improve pathjoin

Add an ability to treat single '/' as no-op.
Ignore duplicate '/' in pathjoin.

Closes #2968

src/lua/fio.lua

@@ -212,16 +212,15 @@ fio.pathjoin = function(path, ...)
         if sp == nil then
             error("Undefined path part")
         end
-        if sp == '' or sp == '/' then
+        if sp == '' then
             error("Empty path part")
         end
-        if string.match(sp, '^/') ~= nil then
-            sp = string.gsub(sp, '^/', '')
-        end
+
         if sp ~= '' then
             path = path .. '/' .. sp
         end
     end
+    path = string.gsub(path, "/+",'/')
     if string.match(path, '/$') ~= nil and #path > 1 then
         path = string.gsub(path, '/$', '')
     end

test/app/fio.result

@@ -44,6 +44,14 @@ fio.pathjoin('/', '/cde')
 ---
 - /cde
 ...
+fio.pathjoin('/a', '/')
+---
+- /a
+...
+fio.pathjoin('abc', 'awdeq///qweqwqwe///', "//asda//")
+---
+- abc/awdeq/qweqwqwe/asda
+...
 -- basename
 st, err = pcall(fio.basename, nil)
 ---

test/app/fio.test.lua

@@ -15,6 +15,8 @@ fio.pathjoin('abc', 'cde')
 fio.pathjoin('/', 'abc')
 fio.pathjoin('abc/', '/cde')
 fio.pathjoin('/', '/cde')
+fio.pathjoin('/a', '/')
+fio.pathjoin('abc', 'awdeq///qweqwqwe///', "//asda//")
 
 -- basename
 st, err = pcall(fio.basename, nil)