weston: Add set up SIGUSR1 blocking early using pthread_sigmask()

Xwayland block SIGUSR1 signal for handling this signal. However, if some
weston plugins creates additional threads before xwayland is loaded,
this signal get delivered these threads and causes weston quit.
Therefore, we should set up SIGUSR1 blocking early so that these threads
can inherit the setting when created.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>