signal: Deprecate additional legacy signal handling functions

signal: Deprecate additional legacy signal handling functions

This needs a few test adjustments: In some cases, sigignore was
used for convenience (replaced with xsignal with SIG_IGN).  Tests
for the deprecated functions need to disable
-Wdeprecated-declarations, and for the sigmask deprecation,
-Wno-error.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>