init: Require explicit -asmap filename

Currently, if `-asmap` is specified without a filename, bitcoind tries to load
`ip_asn.map` data file.

This change now requires `-asmap=ip_asn.map` or another filename to be
specified explicitly.

The change is intended to make behavior of the option explicit avoid confusion
reported https://github.com/bitcoin/bitcoin/issues/33386 where documentation
specifies a default file which is not actually loaded by default. It was
originally implemented in
https://github.com/bitcoin/bitcoin/pull/33631#issuecomment-3410302383 and
various alternatives are discussed there.

Co-authored-by: Fabian Jahr <fjahr@protonmail.com>

doc/files.md

@@ -66,7 +66,6 @@ Subdirectory       | File(s)               | Description
 `./`               | `debug.log`           | Contains debug information and general logging generated by `bitcoind` or `bitcoin-qt`; can be specified by `-debuglogfile` option
 `./`               | `fee_estimates.dat`   | Stores statistics used to estimate minimum transaction fees required for confirmation
 `./`               | `guisettings.ini.bak` | Backup of former [GUI settings](#gui-settings) after `-resetguisettings` option is used
-`./`               | `ip_asn.map`          | IP addresses to Autonomous System Numbers (ASNs) mapping used for bucketing of the peers; path can be specified with the `-asmap` option
 `./`               | `mempool.dat`         | Dump of the mempool's transactions
 `./`               | `onion_v3_private_key` | Cached Tor onion service private key for `-listenonion` option
 `./`               | `i2p_private_key`     | Private key that corresponds to our I2P address. When `-i2psam=` is specified the contents of this file is used to identify ourselves for making outgoing connections to I2P peers and possibly accepting incoming ones. Automatically generated if it does not exist.