[Ada] replace call to bzero in terminals.c by call to memset

[Ada] replace call to bzero in terminals.c by call to memset

bzero is marked as legacy in POSIX.1-2001, and using it triggers a
deprecation warnings on some systems such as QNX. This change adjusts
the one place where we use it in terminals.c to use memset instead.
This, in turns, allows us to get rid of a hack for HP/UX and Solaris.

gcc/ada/

	* terminals.c: Remove bzero #define on HP/UX or Solaris
	platforms.
	(child_setup_tty): Replace bzero call by equivalent call to
	memset.