struct sigcontext and ucontext_t/mcontext_t are defined in such
a way that both (ie struct sigcontext and ucontext_t) can be
passed on to sigreturn. The signal handler is still given a
ucontext_t for maximum flexibility.
For backward compatibility sigreturn restores the state for the
alternate signal stack from sigcontext.sc_onstack and not from
ucontext_t.uc_stack. A good way to determine which value the
application has set and thus which value to use, is still open
for discussion.
NOTE: This change should only affect those binaries that use
sigcontext and/or ucontext_t. In the source tree itself
this is only doscmd. Recompilation is required for those
applications.
This commit also fixes a lot of style bugs without hopefully
adding new ones.
NOTE: struct sigaltstack.ss_size now has type size_t again. For
some reason I changed that into unsigned int.
Parts submitted by: bde
sigaltstack bug found by: bde
c5c6b7b38e