Add -Wimplicit-fallthrough to CFLAGS and CXXFLAGS

Use it at level 4, a bit more restrictive than the default level, and
tweak our commanding comments to FALLTHROUGH.

(However, leave zic.c alone, since it's external code; to avoid the
warnings that would appear there, change CFLAGS for that file in the
Makefile.)

Author: Julien Rouhaud <rjuju123@gmail.com>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20200412081825.qyo5vwwco3fv4gdo@nol
Discussion: https://postgr.es/m/flat/E1fDenm-0000C8-IJ@gemulon.postgresql.org

src/interfaces/ecpg/pgtypeslib/interval.c

@@ -391,7 +391,7 @@ DecodeInterval(char **field, int *ftype, int nf,	/* int range, */
 					tmask = DTK_M(TZ);
 					break;
 				}
-				/* FALL THROUGH */
+				/* FALLTHROUGH */
 
 			case DTK_DATE:
 			case DTK_NUMBER:

src/interfaces/libpq/fe-secure.c

@@ -375,7 +375,7 @@ retry_masked:
 				REMEMBER_EPIPE(spinfo, true);
 
 #ifdef ECONNRESET
-				/* FALL THRU */
+				/* FALLTHROUGH */
 
 			case ECONNRESET:
 #endif