utils: drop strtok_r from envlist_parse

The problem is that mingw 4.9.1 fails to compile the code with the
following warning:

/mingw/include/string.h:88:9: note: previous declaration of 'strtok_r'
was here
   char *strtok_r(char * __restrict__ _Str,
                  const char * __restrict__ _Delim,
                  char ** __restrict__ __last);
/include/sysemu/os-win32.h:83:7: warning: redundant redeclaration of
   'strtok_r' [-Wredundant-decls]
   char *strtok_r(char *str, const char *delim, char **saveptr);

The problem is that compiles just fine on previous versions of mingw.
Compiler version check here is not a good idea. Though fortunately
strtok_r is used only once in the code and we could simply rewrite
the code without it.

Signed-off-by: Olga Krishtal <okrishtal@parallels.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
