MFC 302900,302902,302921,303461,304009:
Add a mask of optional ptrace() events.

302900:
Add a test for user signal delivery.

This test verifies we get the correct ptrace event details when a signal
is posted to a traced process from userland.

302902:
Add a mask of optional ptrace() events.

ptrace() now stores a mask of optional events in p_ptevents. Currently
this mask is a single integer, but it can be expanded into an array of
integers in the future.

Two new ptrace requests can be used to manipulate the event mask:
PT_GET_EVENT_MASK fetches the current event mask and PT_SET_EVENT_MASK
sets the current event mask.

The current set of events include:

• PTRACE_EXEC: trace calls to execve().
• PTRACE_SCE: trace system call entries.
• PTRACE_SCX: trace syscam call exits.
• PTRACE_FORK: trace forks and auto-attach to new child processes.
• PTRACE_LWP: trace LWP events.

The S_PT_SCX and S_PT_SCE events in the procfs p_stops flags have
been replaced by PTRACE_SCE and PTRACE_SCX. PTRACE_FORK replaces
P_FOLLOW_FORK and PTRACE_LWP replaces P2_LWP_EVENTS.

The PT_FOLLOW_FORK and PT_LWP_EVENTS ptrace requests remain for
compatibility but now simply toggle corresponding flags in the
event mask.

While here, document that PT_SYSCALL, PT_TO_SCE, and PT_TO_SCX both
modify the event mask and continue the traced process.

302921:
Rename PTRACE_SYSCALL to LINUX_PTRACE_SYSCALL.

303461:
Note that not all optional ptrace events use SIGTRAP.

New child processes attached due to PTRACE_FORK use SIGSTOP instead of
SIGTRAP. All other ptrace events use SIGTRAP.

304009:
Remove description of P_FOLLOWFORK as this flag was removed.