sh: fix debug trap failure to process signals before return to user

sh: fix debug trap failure to process signals before return to user

When responding to a debug trap (breakpoint) in userspace, the
kernel's trap handler raised SIGTRAP but returned from the trap via a
code path that ignored pending signals, resulting in an infinite loop
re-executing the trapping instruction.

Signed-off-by: Rich Felker <dalias@libc.org>