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/release-notes-33770.md

@@ -0,0 +1,4 @@
+`-asmap` requires explicit filename
+-----------------------------------
+
+In previous releases, if `-asmap` was specified without a filename, this would try to load an `ip_asn.map` data file. Now loading an asmap file requires an explicit filename like `-asmap=ip_asn.map`. This change was made to make the option easier to understand, because it was confusing for there to be a default filename not actually loaded by default (https://github.com/bitcoin/bitcoin/issues/33386). Also this change makes the option more future-proof, because in upcoming releases, specifying `-asmap` will load embedded asmap data instead of an external file (https://github.com/bitcoin/bitcoin/pull/28792).