Settings given as files: the _FILE form of every setting (closes #87)

Every setting X may instead be given as a file that X_FILE names, read
once at start: its contents, less one trailing newline, are the value,
checked as X would be. X and X_FILE both set, or a file that cannot be
read, stops the start with a message naming the variable. The logged
settings name the file, and mask a token read from one.
SWWAF_LOG_REMOTE_TLS_CA_FILE, whose value is a file already, has no
_FILE form. The health check reads only SWWAF_LISTEN_ADDR and
SWWAF_UPSTREAM_URL, so no other setting or file can fail it.

Judgement call: an invalid value read from a file is named as X, not X_FILE.
Rule suppressed: gosec G304 on reading the named file, as for the CA file.

Model: opus-5-5

SPEC.md

@@ -32,7 +32,7 @@ from a directory of hand-editable text files.
 - Defence against traffic floods that saturate the host's network link. That
   needs help upstream of the host.
 - A web UI or a configuration file. Settings are environment variables. Apart
-  from settings given as files (the `_FILE` form of any setting, such as
+  from settings given as files (the `_FILE` form of a setting, such as
   `SWWAF_ADMIN_TOKEN_FILE`, and `SWWAF_LOG_REMOTE_TLS_CA_FILE`), its own state
   files and the lookup database, the only files read are the rule files, which
   hold one regex per line and nothing more elaborate.
@@ -298,7 +298,8 @@ it.
 - A list set to an empty value is an empty list, and replaces the default.
 - Every setting may instead be given as a file holding the value, named by the
   setting's name with `_FILE` added, such as `SWWAF_ADMIN_TOKEN_FILE`, for
-  secrets and long lists.
+  secrets and long lists. `SWWAF_LOG_REMOTE_TLS_CA_FILE`, whose value names a
+  file already, has no `_FILE` form.
 - Settings, including those given as files, are read once at start; changing one
   means restarting the container. The files `smallwebwaf` watches while it runs
   are its state files, its rule files and the lookup database.