Fix incorrect ordering of operations in pg_resetwal and pg_rewind.

Commit c37b3d08c dropped its added GetDataDirectoryCreatePerm call into
the wrong place in pg_resetwal.c, namely after the chdir to DataDir.
That broke invocations using a relative path, as reported by Tushar Ahuja.
We could have left it where it was and changed the argument to be ".",
but that'd result in a rather confusing error message in event of a
failure, so re-ordering seems like a better solution.

Similarly reorder operations in pg_rewind.c.  The issue there is that
it doesn't seem like a good idea to do any actual operations before the
not-root check (on Unix) or the restricted token acquisition (on Windows).
I don't know that this is an actual bug, but I'm definitely not convinced
that it isn't, either.

Assorted other code review for c37b3d08c and da9b580d8: fix some
misspelled or otherwise badly worded comments, put the #include for
<sys/stat.h> where it actually belongs, etc.

Discussion: https://postgr.es/m/aeb9c3a7-3c3f-a57f-1a18-c8d4fcdc2a1f@enterprisedb.com

src/include/common/file_perm.h

@@ -1,6 +1,6 @@
 /*-------------------------------------------------------------------------
  *
- * File and directory permission constants
+ * File and directory permission definitions
  *
  *
  * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
@@ -13,6 +13,8 @@
 #ifndef FILE_PERM_H
 #define FILE_PERM_H
 
+#include <sys/stat.h>
+
 /*
  * Mode mask for data directory permissions that only allows the owner to
  * read/write directories and files.