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

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.