Collection of typo fixes.

Use "a" and "an" correctly, mostly in comments. Two error messages were
also fixed (they were just elogs, so no translation work required). Two
function comments in pg_proc.h were also fixed. Etsuro Fujita reported one
of these, but I found a lot more with grep.

Also fix a few other typos spotted while grepping for the a/an typos.
For example, "consists out of ..." -> "consists of ...". Plus a "though"/
"through" mixup reported by Euler Taveira.

Many of these typos were in old code, which would be nice to backpatch to
make future backpatching easier. But much of the code was new, and I didn't
feel like crafting separate patches for each branch. So no backpatching.

src/include/port/atomics/generic-gcc.h

@@ -117,7 +117,7 @@ typedef struct pg_atomic_uint64
 static inline bool
 pg_atomic_test_set_flag_impl(volatile pg_atomic_flag *ptr)
 {
-	/* NB: only a acquire barrier, not a full one */
+	/* NB: only an acquire barrier, not a full one */
 	/* some platform only support a 1 here */
 	return __sync_lock_test_and_set(&ptr->value, 1) == 0;
 }