USE_SEIZE: fix detaching from stopped processes

USE_SEIZE: fix detaching from stopped processes

V3: split SEIZE/!SEIZE code paths to reduce confusion.
Extensively comment every possible case.
Verified that all tests/detach* tests work in both SEIZE and !SEIZE
cases.

* strace.c (detach): If PTRACE_SEIZE API is in use, stop the tracee
using PTRACE_INTERRUPT instead of sending it a SIGSTOP.
In a subsequent waitpid loop, correctly wait and suppress SIGSTOP
on detach if PTRACE_INTERRUPT wasn't used, or wait for any ptrace
stop and detach without suppressing signals.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>