path-conversion: Introduce ability to switch off conversion.

When calling windows native apps from MSYS2, the runtime tries to convert
commandline arguments by a specific set of rules. See [MinGW wiki]
(http://www.mingw.org/wiki/Posix_path_conversion).

If the user does not want that behavior on a big scale, e.g. inside a bash
script, the user can now set the the environment variable
`MSYS_NO_PATHCONV` when calling native windows commands.

Signed-off-by: 마누엘 <nalla@hamal.uberspace.de>

winsup/cygwin/msys2_path_conv.cc

@@ -348,6 +348,14 @@ path_type find_path_start_and_type(const char** src, int recurse, const char* en
         return NONE;
     }
 
+    /*
+     * Skip path mangling when environment indicates it.
+     */
+    const char *no_pathconv = getenv ("MSYS_NO_PATHCONV");
+
+    if (no_pathconv)
+      goto skip_p2w;
+
     /*
      * Prevent Git's :file.txt and :/message syntax from beeing modified.
      */