find: Fix -printf

Each -printf invocation created a memstream, then freed the underlying
buffer without closing the stream, resulting in a segfault on exit
when libc tried to flush all the streams.

find: Fix -printf

Each -printf invocation created a memstream, then freed the underlying
buffer without closing the stream, resulting in a segfault on exit
when libc tried to flush all the streams. Drop the memstream, which
isn't really needed.

Furthermore:

* Change escape() to return char * instead of const char *
* Simplify the implementation of %h
* Fix %M, which printed an extra space
* Implement %l correctly
* Implement %Y and %y
* Add tests for everything except %S

Fixes: 7b9c912c41f4
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D51776

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