Define mirror targets one way: remote.<name>.<field>

There were two spellings for the same thing -- a flat
remoteurl/remotekey/remotetype/remotevisibility set for a single server,
and remote.<name>.* blocks for several. The flat one is gone; every
target, including a lone one, is now a named block with the fields url,
key, type and visibility.

An existing ~/.mgshrc is converted on the next start. Only the key is
rewritten, so values, comments, alignment, commented-out lines and the
file's 0600 mode survive untouched, and mgsh prints each rename rather
than doing it quietly. The target is named "public", which is what the
old settings called the git remote they created, so a converted setup
keeps pushing to the same place under the same remote name. A file that
carries both spellings keeps what the new one says.

The environment follows the same shape: MGSH_REMOTEURL and friends are
replaced by MGSH_REMOTE_<NAME>_<FIELD>, so MGSH_REMOTE_GITLAB_KEY sets
remote.gitlab.key. The field is read from the end of the variable name,
which leaves target names free to contain underscores.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

version.txt

@@ -1 +1 @@
-4.0.40
+4.0.42