daemon: initialize mask_orig with sigemptyset()

consolidation of variable declarations and initializations in previous
commit allowed me to detect that one of the signal masks is not properly
initialized with sigemptyset (as man 3 sigsetops demands)

Reviewed by: kevans
Pull Request: https://github.com/freebsd/freebsd-src/pull/669

(cherry picked from commit 84866cefdfb78488543056173aea78748b77159b)