1.0.15.2: Add binding to fcntl's struct flock in SB-POSIX.

1.0.15.2: Add binding to fcntl's struct flock in SB-POSIX.

File by file breakdown:

* contrib/sb-posix/constants.lisp

Add an internal ALIEN-FLOCK structure.

* contrib/sb-posix/defpackage.lisp

Export the FLOCK class the accessors.

* contrib/sb-posix/interface.lisp

Define a FLOCK "protocol class", add a path through FCNTL that accepts
FLOCK instances.

* sb-posix/posix-tests.lisp

Test two uses of exclusive (F_WRLCK) locks.

* contrib/sb-posix/sb-posix.texinfo

Add documentation for the FLOCK class to the manual.