When LittleFS.begin() or SDFS.begin() is called after the filesystem is
already mounted, don't unmount/remount. When an unmount happens, all old
Files become invalid (but the core doesn't know this), so you would end
up with random crashes in FS code.
Now, check for _mounted, and if so just return immediately from begin().
The O_CREAT/etc. flags on the Pico are a full 32-bits in size, but the
core was generating a flag for SdFat using an 8-bit type, so all the
O_CREAT, O_TRUNC, O_APPEND, etc. flags got cut off.
Fix the flag size.
Fixes#214
