Make related code compilable with frontend

This commit makes the code around keyring, principal keys and WAL
encryption compilable with frontend tools. Namely:
- Hide everything that isn't compatible and of no use behind
  '#ifndef FRONTEND'
- Redefine code that is needed in both versions but should have
  different code. E.g. error handling, file descriptors and locks
- Make use of frontend lists instead of backend ones where needed.

Makefile.in

@@ -36,8 +36,8 @@ src$(MAJORVERSION)/access/pg_tde_rewrite.o \
 src$(MAJORVERSION)/access/pg_tdeam_handler.o \
 src/access/pg_tde_ddl.o \
 src/access/pg_tde_xlog.o \
+src/access/pg_tde_xlog_encrypt.o \
 src/transam/pg_tde_xact_handler.o \
-src/keyring/keyring_config.o \
 src/keyring/keyring_curl.o \
 src/keyring/keyring_file.o \
 src/keyring/keyring_vault.o \

meson.build

@@ -32,11 +32,11 @@ pg_tde_sources = files(
         src_version / 'access/pg_tde_visibilitymap.c',
         'src/access/pg_tde_ddl.c',
         'src/access/pg_tde_xlog.c',
+        'src/access/pg_tde_xlog_encrypt.c',
 
         'src/encryption/enc_tde.c',
         'src/encryption/enc_aes.c',
 
-        'src/keyring/keyring_config.c',
         'src/keyring/keyring_curl.c',
         'src/keyring/keyring_file.c',
         'src/keyring/keyring_vault.c',