Add FILE_RETRY_ERRNO() definition, to be used in sopen()/lock() loops

Note: EBUSY was added to the list of errno values that should cause a retry
as observed on Vertrauen, running sbbs-linux on a Samba mount, opening
locked files can return failure (-1) with errno = EBUSY, e.g.
evnt BBS Events !ERROR 16 (Device or resource busy) in logon.cpp line 616 (logonstats) reading "system stats" access=0

This was apparently resolved by adding EBUSY (16) the values that trigger an sopen()
retry (i.e. from [f]nopen()).

src/xpdev/filewrap.h

@@ -144,6 +144,9 @@
 #define CLOSE_OPEN_FILE(x)	do { if((x) >= 0)    { close(x);  (x) = -1;   } } while(0)
 #define FCLOSE_OPEN_FILE(x)	do { if((x) != NULL) { fclose(x); (x) = NULL; } } while(0)
 
+// These errno values should trigger sopen() and lock() retries, within limits
+#define FILE_RETRY_ERRNO(x) ((x)==EACCES || (x)==EAGAIN || (x)==EDEADLOCK || (x)==EBUSY)
+
 /**************/
 /* Prototypes */
 /**************/