Refactor code in charge of running shell-based recovery commands

The code specific to the execution of archive_cleanup_command,
recovery_end_command and restore_command is moved to a new file named
shell_restore.c.  The code is split into three functions:
- shell_restore(), that attempts the execution of a shell-based
restore_command.
- shell_archive_cleanup(), for archive_cleanup_command.
- shell_recovery_end(), for recovery_end_command.

This introduces no functional changes, with failure patterns and logs
generated in consequence being the same as before (one case actually
generates one less DEBUG2 message "could not restore" when a restore
command succeeds but the follow-up stat() to check the size fails, but
that only matters with a elevel high enough).

This is preparatory work for allowing recovery modules, a facility
similar to archive modules, with callbacks shaped similarly to the
functions introduced here.

Author: Nathan Bossart
Reviewed-by: Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/20221227192449.GA3672473@nathanxps13

src/include/access/xlogarchive.h

@@ -20,8 +20,6 @@
 extern bool RestoreArchivedFile(char *path, const char *xlogfname,
 								const char *recovername, off_t expectedSize,
 								bool cleanupEnabled);
-extern void ExecuteRecoveryCommand(const char *command, const char *commandName,
-								   bool failOnSignal, uint32 wait_event_info);
 extern void KeepFileRestoredFromArchive(const char *path, const char *xlogfname);
 extern void XLogArchiveNotify(const char *xlog);
 extern void XLogArchiveNotifySeg(XLogSegNo segno, TimeLineID tli);
@@ -32,4 +30,9 @@ extern bool XLogArchiveIsReady(const char *xlog);
 extern bool XLogArchiveIsReadyOrDone(const char *xlog);
 extern void XLogArchiveCleanup(const char *xlog);
 
+extern bool shell_restore(const char *file, const char *path,
+						  const char *lastRestartPointFileName);
+extern void shell_archive_cleanup(const char *lastRestartPointFileName);
+extern void shell_recovery_end(const char *lastRestartPointFileName);
+
 #endif							/* XLOG_ARCHIVE_H */