This fix is for compatibility with lwip_select(). It moves the lock to
UART which is the only VFS driver which is implemented and is not
"thread-safe".
When using multiple sockets, some of them could be incorrectly removed
from the fd_sets before passing these fd_sets forward to the socket
select function.
Closes https://github.com/espressif/esp-idf/issues/1987
vfs_uart used same locks for read and write operations on a single UART.
If read operation was blocking (i.e. carried out via UART driver), the
lock was held by reading task until it received a line. During this time,
other tasks trying to write to the same UART would get blocked.
This change introduces separate read/write locks, and adds a test.
Another vfs_uart test if fixed (it was disabled since the
CONFIG_NEWLIB_STDOUT_ADDCR option was removed).
Previously opendir("/data") would fail if filesystem with "data" prefix
was registered in VFS, while opendir("/data/") would succeed.
This change fixes handling for the former case and adds relevant tests.
