Rename pg_rewind's copy_file_range() to avoid conflict with new linux syscall.

Upcoming versions of glibc will contain copy_file_range(2), a wrapper
around a new linux syscall for in-kernel copying of data ranges. This
conflicts with pg_rewinds function of the same name.

Therefore rename pg_rewinds version. As our version isn't a generic
copying facility we decided to choose a rewind specific function name.

Per buildfarm animal caiman and subsequent discussion with Tom Lane.

Author: Andres Freund
Discussion:
    https://postgr.es/m/20180103033425.w7jkljth3e26sduc@alap3.anarazel.de
    https://postgr.es/m/31122.1514951044@sss.pgh.pa.us
Backpatch: 9.5-, where pg_rewind was introduced

src/bin/pg_rewind/copy_fetch.c

@@ -156,7 +156,7 @@ recurse_dir(const char *datadir, const char *parentpath,
  * If 'trunc' is true, any existing file with the same name is truncated.
  */
 static void
-copy_file_range(const char *path, off_t begin, off_t end, bool trunc)
+rewind_copy_file_range(const char *path, off_t begin, off_t end, bool trunc)
 {
 	char		buf[BLCKSZ];
 	char		srcpath[MAXPGPATH];
@@ -222,7 +222,7 @@ copy_executeFileMap(filemap_t *map)
 				break;
 
 			case FILE_ACTION_COPY:
-				copy_file_range(entry->path, 0, entry->newsize, true);
+				rewind_copy_file_range(entry->path, 0, entry->newsize, true);
 				break;
 
 			case FILE_ACTION_TRUNCATE:
@@ -230,7 +230,8 @@ copy_executeFileMap(filemap_t *map)
 				break;
 
 			case FILE_ACTION_COPY_TAIL:
-				copy_file_range(entry->path, entry->oldsize, entry->newsize, false);
+				rewind_copy_file_range(entry->path, entry->oldsize,
+									   entry->newsize, false);
 				break;
 
 			case FILE_ACTION_CREATE:
@@ -257,7 +258,7 @@ execute_pagemap(datapagemap_t *pagemap, const char *path)
 	while (datapagemap_next(iter, &blkno))
 	{
 		offset = blkno * BLCKSZ;
-		copy_file_range(path, offset, offset + BLCKSZ, false);
+		rewind_copy_file_range(path, offset, offset + BLCKSZ, false);
 		/* Ok, this block has now been copied from new data dir to old */
 	}
 	pg_free(iter);