Move the internal OpenPGP implementation to its own file.

Split the internal OpenPGP implementation into the bits that are
needed by a new OpenPGP backend like Sequoia, and the bits that are
not needed by another OpenPGP backend.

Move most of the functionality in rpmio/rpmpgp.c into
rpmio/rpmpgp_internal.c.

Leave pgpValStr, and pgpIdentItem, which are used for printing and
needn't be reimplemented by other backends, and pgpReadPkts, which is
just a thin wrapper around pgpParsePkts, and which uses an internal
rpm function that a new backend shouldn't have to worry about
emulating or even calling.

Move the symbol tables, which are used by pgpValStr, pgpIdentItem, and
the internal OpenPGP implementation to rpmio/rpmpgp.h.  These are
common to all implementations.

Fixes #2000.

rpmio/Makefile.am

@@ -20,11 +20,11 @@ usrlib_LTLIBRARIES = librpmio.la
 librpmio_la_SOURCES = \
 	argv.c base64.c digest.h digest.c expression.c macro.c \
 	rpmhook.c rpmio.c rpmlog.c rpmmalloc.c rgetopt.c \
-	rpmpgp.c rpmsq.c rpmsw.c url.c \
+	rpmpgp.c rpmpgp.h rpmsq.c rpmsw.c url.c \
 	rpmio_internal.h rpmhook.h rpmvercmp.c rpmver.c \
 	rpmstring.c rpmfileutil.c rpmglob.c \
 	rpmkeyring.c rpmstrpool.c rpmmacro_internal.h \
-	rpmlua.c rpmlua.h lposix.c lposix.h
+	rpmlua.c rpmlua.h lposix.c lposix.h rpmpgp_internal.c
 
 if WITH_OPENSSL
 librpmio_la_SOURCES += digest_openssl.c