mesh-reads: read a node's files and accounts as an account that holds no secret (hq ADR 0310, issue 418)

Reading a file, searching one, listing a directory, the owner and mode of each
component of a path and looking up an account went through the login shell's
execute as the operator's account, which sees the operator's keys and the
agent's credentials. This module holds node-reads on every node and runs as
mesh-reads (no login, no group beside its own). It refuses the node's secret
places by name, judging the file actually opened rather than the name asked,
so a flipped link reaches nothing refused; withholds any line shaped like a
secret through mesh-sdk's outward rule and every line of a BEGIN/END block;
and bounds every answer. Replaces mesh-catalog #244 (reviews 55), whose first
commit carried a built binary.

.gitignore

@@ -19,3 +19,5 @@ modules/mesh-issues/cmd/mesh-issues/mesh-issues
 modules/mounts/cmd/mounts/mounts
 modules/mesh-knowledge/cmd/mesh-knowledge/mesh-knowledge
 modules/mesh-knowledge/mesh-knowledge
+modules/mesh-reads/mesh-reads
+modules/mesh-reads/cmd/mesh-reads/mesh-reads