Previously, with -p, the specified directory and all of its parents
would be 0777&~filemask (regardless of the -m flag). POSIX says parent
directories must created as (0300|~filemask)&0777, and of course if -m
is set, the specified directory should be created with those
permissions.
Additionally, POSIX says that for symbolic_mode strings, + and - should
be interpretted relative to a default mode of 0777 (not 0).
Without -p, previously the directory would be created first with
0777&~filemask (before a chmod), but POSIX says that the directory shall
at no point in time have permissions less restrictive than the -m mode
argument.
Rather than dealing with mkdir removing the filemask bits by calling
chmod afterward, just clear the umask and remove the bits manually.
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