sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

tslog.h: Wrap in #ifdef _KERNEL

This is not intended to be used from outside of the kernel; in
particular, the boot loader has its own version of tslog.

Add userland boot profiling to TSLOG

On kernels compiled with 'options TSLOG', record for each process ID:
* The timestamp of the fork() which creates it and the parent
process ID,
* The first path passed to execve(), if any,
* The first path resolved by namei, if any, and
* The timestamp of the exit() which terminates the process.

Expose this information via a new sysctl, debug.tslog_user.

On kernels lacking 'options TSLOG' (the default), no information is
recorded and the sysctl does not exist.

Note that recording namei is needed in order to obtain the names of
rc.d scripts being launched, as the rc system sources them in a
subshell rather than execing the scripts.

With this commit it is now possible to generate flamecharts of the
entire boot process from the start of the loader to the end of
/etc/rc. The code needed to perform this processing is currently
found in github: https://github.com/cperciva/freebsd-boot-profiling

Reviewed by: mhorne
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D32493

Wrap includes in sys/tslog.h with #ifdef TSLOG.

This is necessary because some non-kernel code #defines _KERNEL and then
includes kernel headers; as a result, it was getting conflicting versions
of curthread and curproc. Non-kernel code should probably refrain from
defining _KERNEL, but for now hiding these indirect inclusions fixes the
build.

Reported by: Michael Butler, Herbert J. Skuhra

Code for recording timestamps of events, especially function entries/exits.
This is a very primitive system, intended for use in measuring performance
during the early system boot, before more sophisticated tools like DTrace
or infrastructure like kernel memory allocation and mutexes are available.

Because this code records pointers to strings rather than copying strings
(in order to keep the memory usage more manageable), if a kernel module is
unloaded after logging an event, Bad Things can happen. Users are advised
to not do that.

Since cycle counts from the early kernel boot are used as an initial entropy
source, publishing this information to userland could result in inadequate
entropy being kept private to the kernel RNG. Users are advised to not
enable this on systems with untrusted users.

Discussed on: freebsd-current