Add a pg_lsn data type, to represent an LSN.

Robert Haas and Michael Paquier
pg_lsn macro naming and type behavior revisions.

Change pg_lsn_mi so that it can return negative values when subtracting
LSNs, and clean up some perhaps ill-considered macro names.
Further code review for pg_lsn data type.

Change input function error messages to be more consistent with what is
done elsewhere.  Remove a bunch of redundant type casts, so that the
compiler will warn us if we screw up.  Don't pass LSNs by value on
platforms where a Datum is only 32 bytes, per buildfarm.  Move macros
for packing and unpacking LSNs to pg_lsn.h so that we can include
access/xlogdefs.h, to avoid an unsatisfied dependency on XLogRecPtr.
Switch various builtin functions to use pg_lsn instead of text.

The functions in slotfuncs.c don't exist in any released version,
but the changes to xlogfuncs.c represent backward-incompatibilities.
Per discussion, we're hoping that the queries using these functions
are few enough and simple enough that this won't cause too much
breakage for users.

Michael Paquier, reviewed by Andres Freund and further modified
by me.
Document pg_replslot in storage.sgml.

Per an observation from Amit Kapila.
Fix some missing .gitignore and "make clean" items in ecpg.

Some of the files we optionally link in from elsewhere weren't ignored
and/or weren't cleaned up at "make clean".  Noted while testing on a
machine that needs our version of snprintf.c.
Avoid using dllwrap to build pgevent in Mingw builds.

If this works, we can get rid of configure's support for locating dllwrap
... but let's see what the buildfarm says, first.

Hiroshi Inoue
Remove inappropriate EXPORTS line.

Looks like this gets added later ...