Added API to set mt environment variables.

Details:
- Renamed bli_env_get_nway() -> bli_thread_get_env().
- Added bli_thread_set_env() to allow setting environment variables
  pertaining to multithreading, such as BLIS_JC_NT or BLIS_NUM_THREADS.
- Added the following convenience wrapper routines:
    bli_thread_get_jc_nt()
    bli_thread_get_ic_nt()
    bli_thread_get_jr_nt()
    bli_thread_get_ir_nt()
    bli_thread_get_num_threads()
    bli_thread_set_jc_nt()
    bli_thread_set_ic_nt()
    bli_thread_set_jr_nt()
    bli_thread_set_ir_nt()
    bli_thread_set_num_threads()
- Added #include "errno.h" to bli_system.h.
- This commit addresses issue #140.
- Thanks to Chris Goodyer for inspiring these updates.

frame/include/bli_system.h

@@ -41,6 +41,7 @@
 #include <string.h>
 #include <stdarg.h>
 #include <float.h>
+#include <errno.h>
 
 // Determine if we are on a 64-bit or 32-bit architecture
 #if defined(_M_X64) || defined(__x86_64) || defined(__aarch64__) || \