Made geterr clear the error indicator

Changelog.txt

@@ -1,5 +1,6 @@
 Version 6.4.1
 - Added MKDIR function for BASIC
+- Made _geterror() clear the error number
 
 Version 6.4.0
 - Added getcrc() builtin for Spin2

doc/basic.md

@@ -2333,6 +2333,15 @@ Propeller specific builtin function.
 ```
 Returns the error number `e` corresponding to the last system error. (This is the same as `errno` in C.) This number may be converted to a user-displayable string via `strerror$(e)`.
 
+Note that the error number is not cleared by most library functions, and so is not always a reliable indicator of whether the last call succeeded. For that, one should use the return value of the call itself.
+
+`geterr` resets the error indicator to `EOK`; thus in:
+```
+let r1 = geterr()
+let r2 = geterr()
+```
+`r2` will always be 0 (`EOK`) since there are no intervening system calls between `geterr` calls.
+
 ### GETMS
 
 ```

include/libc/stdlib/errno.c

@@ -3,7 +3,7 @@
 
 static int errno;
 
-int _geterror() { return errno; }
+int _geterror() { int r = errno; errno = 0; return r; }
 int _seterror(int num) { errno = num; if (num) { return -1; } else return 0; }
 
 int *_geterrnoptr() { return &errno; }