Convert FatFS date/time to Unix timestamp in SFTP #1496

Job	Run time
create_matrix	4s
Build wolfssl (ubuntu-latest, v5.9.1-stable)	4s
Build wolfssl (ubuntu-latest, master)	3s
Build wolfssl (ubuntu-latest, v5.9.0-stable)	5s
Build and test wolfsshd (ubuntu-latest, v5.9.0-stable)	2m 18s
Build and test wolfsshd (ubuntu-latest, v5.9.1-stable)	2m 46s
Build and test wolfsshd (ubuntu-latest, master)	2m 20s
	7m 40s

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