Split mutex.c and .h each into three files

Split mutex.c and .h each into three files

Create separate header and source files for the struct fio_mutex
functions, for the struct fio_rwlock functions and for the helper
functions for initializing process shared mutexes and condition
variables. Replace an #include directive by a forward declaration
in fio.h. Minimize the #include directives in mutex.c, rwlock.c
and pshared.c. This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>