Add an kinfo sysctl to retrieve signal trampoline location for the
given process.

Note that the correctness of the trampoline length returned for ABIs
which do not use shared page depends on the correctness of the struct
sysvec sv_szsigcodebase member, which will be fixed on as-need basis.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
mdoc: sort sections.
Use sysctl KERN_PROC_SIGTRAMP to retrieve the signal trampoline
location for the native amd64 ABI.  This fixes unwinding over the
signal frame after trampoline was moved to the shared page.

The code would be more correct if using sysctl for the target process
instead of inspecting gdb' own trampoline, but the current change is
least intrusive and currently, we always initialize the native ABI
sysvec first, which means that trampoline location for FreeBSD/amd64
ABI is relatively stable.

Similar change will benefit libunwind.

Analyzed by:	avg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Create /var/cache with mode 0755 instead of 0750.

This directory is used by many third party applications and having
permission 0750 makes it impossible to drop group privileges.

MFC after:	3 days