global variable-initializer macros XLAL_INIT_MEM() and XLAL_INIT_DECL()

- XLAL_INIT_MEM(x) sets the memory of variable 'x' to zero
- removed all (identical) previous local definitions of that macro
- added C99 convenience-macro XLAL_INIT_DECL(x) for declare+initialize a
  variable x, as in: INT4 XLAL_INIT_DECL(myvar);
- refs #1091
Replaced all empty_XXX initializers by XLAL_INIT_MEM()/XLAL_INIT_DECL() macros

- closes #1091