lock() for Unix now sets the appropriate lock type (shareable or exclusive) based on how the file was initially opened.

Added compiler error when __unix__ is defined, but not _POSIX_THREADS.

Eliminated unnecessary setting of flock.l_pid (only used on F_GETLK).

Removed definition of SH_DENYRD for Unix targets (unused in SBBS).

Beautified (indented) the code.

Created new make_data_dirs() function to create data directory and hard-coded sub-directories if they don't already exist.