system(): on command == NULL, return nonzero if shell exists

From man system.3:

   The return value of system() is one of the following:

   *  If command is NULL, then a nonzero value if a shell is
      available, or 0 if no shell is available.

This makes relibc system() conform to POSIX in that regard (and also adds a test).

merged

mentioned in commit acbe3236