fix: keep the mirror readable under a umask that masks the owner's read bit

The umask handling added for group access cleared only the group bits and left
owner and other to the environment. A container whose umask carries 0400 then
produces mirror directories of mode 0300: writable and enterable, unreadable to
the very run that created them, and unreadable to anything serving the share.

Clear the owner read and execute bits from the umask as well. The `other` bits
stay where the environment puts them, because whether the mirror is
world-readable is a real policy question; being able to read a directory the
process itself just created is not.

Files were never exposed to this: mkstemp sets 0600 outright and copy2 takes
the source file's mode, both ignoring the umask.

README.md

@@ -63,9 +63,14 @@ Everything written into the mirror is made group-readable, and its directories
 group-traversable, so the mirror can be read back by whatever serves it. Neither
 writer does that unaided: the temporary file an encode renames into place is
 created `0600` regardless of the umask, and a straight copy of an existing MP3
-inherits the mode of a source file in a library this tool does not own. Only the
-group bits are touched; whether the mirror is world-readable stays with the
-umask, as does the ownership.
+inherits the mode of a source file in a library this tool does not own.
+
+Directories are handled by clearing the owner and group read/execute bits from
+the process umask, once, at startup. Owner as well as group, because a umask
+carrying `0400` produces directories of mode `0300` — writable and enterable,
+unreadable to the very run that created them. The `other` bits are left where
+the umask puts them: whether the mirror is world-readable is a genuine policy
+question, and so is its ownership.
 
 Mirror files written before this existed are topped up on the next pass. Their
 mtimes are correct, so nothing else would revisit them — and they are not