fix types of ansi_c/sysio to more closely match C ABI

also fixes some instances of using C library functions when there are
nim alternatives available

lib/pure/memfiles.nim

@@ -257,12 +257,10 @@ type MemSlice* = object  ## represent slice of a MemFile for iteration over deli
   data*: pointer
   size*: int
 
-proc c_memcpy(a, b: pointer, n: int) {.importc: "memcpy", header: "<string.h>".}
-
 proc `$`*(ms: MemSlice): string {.inline.} =
   ## Return a Nim string built from a MemSlice.
   var buf = newString(ms.size)
-  c_memcpy(addr(buf[0]), ms.data, ms.size)
+  copyMem(addr(buf[0]), ms.data, ms.size)
   buf[ms.size] = '\0'
   result = buf
 
@@ -329,7 +327,7 @@ iterator lines*(mfile: MemFile, buf: var TaintedString, delim='\l', eat='\r'): T
 
   for ms in memSlices(mfile, delim, eat):
     buf.setLen(ms.size)
-    c_memcpy(addr(buf[0]), ms.data, ms.size)
+    copyMem(addr(buf[0]), ms.data, ms.size)
     buf[ms.size] = '\0'
     yield buf
 

lib/pure/os.nim

@@ -61,7 +61,7 @@ proc osErrorMsg*(): string {.rtl, extern: "nos$1", deprecated.} =
           result = $msgbuf
           if msgbuf != nil: localFree(msgbuf)
   if errno != 0'i32:
-    result = $os.strerror(errno)
+    result = $os.c_strerror(errno)
 
 {.push warning[deprecated]: off.}
 proc raiseOSError*(msg: string = "") {.noinline, rtl, extern: "nos$1",
@@ -114,7 +114,7 @@ proc osErrorMsg*(errorCode: OSErrorCode): string =
           if msgbuf != nil: localFree(msgbuf)
   else:
     if errorCode != OSErrorCode(0'i32):
-      result = $os.strerror(errorCode.int32)
+      result = $os.c_strerror(errorCode.int32)
 
 proc raiseOSError*(errorCode: OSErrorCode; additionalInfo = "") {.noinline.} =
   ## Raises an ``OSError`` exception. The ``errorCode`` will determine the