config: a bad number or true/false setting stops startup (closes #269)

PORT and DNSWATCHER_TLS_EXPIRY_WARNING were read with viper.GetInt,
and DNSWATCHER_DEBUG, DNSWATCHER_MAINTENANCE_MODE and
DNSWATCHER_SEND_TEST_NOTIFICATION with viper.GetBool, which turn a
value that does not parse into 0 or false with no error. Each is now
parsed the way parseInterval parses an interval: PORT must be 1 to
65535, the expiry warning 1 or more days, the true/false settings a
value strconv.ParseBool accepts. Anything else stops startup with an
error naming the variable and the value; empty still means the
default.

Reading the config file moved, unchanged, from buildConfig into
readConfigFile, so buildConfig stays within the linter's length limit.

Model: opus-5-5

README.md

@@ -463,6 +463,16 @@ or empty variable (`DNSWATCHER_DNS_INTERVAL=`) means the default. If either is
 set to anything else, including a bare number or a zero or negative duration,
 dnswatcher refuses to start with an error naming the variable and the value.
 
+**`PORT` and `DNSWATCHER_TLS_EXPIRY_WARNING`** take a whole number: 1 to 65535
+for `PORT`, and a number of days, 1 or more, for
+`DNSWATCHER_TLS_EXPIRY_WARNING`. `DNSWATCHER_DEBUG`,
+`DNSWATCHER_MAINTENANCE_MODE` and `DNSWATCHER_SEND_TEST_NOTIFICATION` take
+`true` or `false`; `1`, `t`, `T`, `TRUE` and `True` also mean true, and `0`,
+`f`, `F`, `FALSE` and `False` also mean false. An unset or empty variable means
+the default. Any other value, such as `PORT=http`,
+`DNSWATCHER_TLS_EXPIRY_WARNING=7d` or `DNSWATCHER_DEBUG=yes`, makes dnswatcher
+refuse to start with an error naming the variable and the value.
+
 **`DNSWATCHER_SENTRY_DSN` reports crashes in HTTP requests to Sentry.** When it
 is set, a panic in an HTTP request handler is sent to Sentry, and the request
 still gets a `500 Internal Server Error` answer. Nothing else is sent to Sentry: