bmaptools: Use df -P for POSIX portable output

[deepcube]

The df -P option forces output as specified by POSIX. Without -P df can output any format it likes. While most output is the same, I've seen long lines split onto multiple lines without -P which causes errors parsing the output in BmapHelpers.py.

Add -P to ensure a known portable output and avoid errors with long lines.

[twoerner]

Can you please add an SoB line? Thanks.

[deepcube]

Can you please add an SoB line? Thanks.

Done

[twoerner]

Great, thank you!