Added shared memory access suport.

- (trie.h) Defined a data structure for multithread synchronization
mtsync_t.
- (trie.h) Added a mtsync_t pointer into arg_t. All threads in the same search
routine will share the same mutex objects.
- (trie.c) Added a new parameter to 'search': maxthreads.
- (starcode.c) Updated calls to function 'search'.
- (starcode.h, starcode.c) Added new parameter to 'tquery' and
'starcode': maxthreads.
- (starcode.h) MAXTHREADS will be a defined constant for now.
- (main-tquery.c, main-starcode.c) Updated calls to functions 'tquery'
and 'starcode'.
- (Makefile) Added compilation flag -pthread.
- Moved all function declarations from trie.c to trie.h.