Throughout, eliminate third argument to path_conv and use new PC_* constants

for second argument.
* path.h: Generalize SYMLINK_* constants to PC_*.
(path_conv): Create a new method.  Fold third argument into second.
* dll_init.cc (dll_list::alloc): Try harder to find space to allocate dll
struct.
(dll_dllcrt0): Don't check sanity if we've already called dll_crt0.
* path.cc (path_conv::check): Don't check for a null or empty path unless
specifically told with a flag setting.
(check_null_empty_path): New function, adapted from macro.
* syscalls.cc (_rename): Use already-determined file attributes rather than
checking again.
* lib/cygwin/cygwin_attach.dll.c (cygwin_attach_dll): Use a static per_process
structure since this is apparently supposed to be zeroed.
* lib/cygwin_crt0.c (cygwin_crt0): Zero per_process structure sent to older
DLLs.

winsup/cygwin/lib/cygwin_crt0.c

@@ -25,6 +25,7 @@ cygwin_crt0 (MainFunc f)
   else			/* Older DLL.  Provide a per_process */
     {
       u = (struct per_process *) alloca (sizeof (*u));
+      memset (u, 0, sizeof (u));
       (void) _cygwin_crt0_common (f, u);
     }
   dll_crt0__FP11per_process (u); 	/* Jump into the dll, never to return */