basic: do not warn in mkdir_p() when parent directory exists

basic: do not warn in mkdir_p() when parent directory exists

This effectively disables warnings about type/mode/ownership of existing
directories when recursively creating parent directories. (Or files. If there's
a file in a place we expect a directory, the code will later try to create
a file and fail. This follows the general pattern where we do (void)mkdir()
if the mkdir() is immediately followed by opening of a file.)

I was recently debugging an issue with the fstab-generator [1], and it says:
'Directory "/tmp" already exists, but has mode 0777 that is too permissive (0644 was requested), refusing.'
which is very specific but totally wrong in this context.
This output was added in 37c1d5e97dbc869edd8fc178427714e2d9428d2b, and I still
think it is worth to do it, because if you actually *do* want the directory, if
there's something wrong, the precise error message will make it much easier to
diagnose. And we can't easily pass the information what failed up the call chain
because there are multiple things we check (ownership, permission mask, type)…
So passing a param whether to warn or not down into the library code seems like
the best solution, despite not being very elegant.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=2051285