vfs_uart: fix write operation blocked by a read

vfs_uart: fix write operation blocked by a read

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).