pgsql: Refactor code in charge of running shell-based recovery commands

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

Branch
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master

Details
-------
https://git.postgresql.org/pg/commitdiff/9a740f81eb02e04179d78f3df2ce671276c27b07

Modified Files
--------------
src/backend/access/transam/Makefile        |   1 +
src/backend/access/transam/meson.build     |   1 +
src/backend/access/transam/shell_restore.c | 175 +++++++++++++++++++++++++++++
src/backend/access/transam/xlog.c          |  37 ++++--
src/backend/access/transam/xlogarchive.c   | 120 +-------------------
src/include/access/xlogarchive.h           |   7 +-
6 files changed, 214 insertions(+), 127 deletions(-)