Tweak markup in _lwp_*(2) manual pages

Use .Rv where possible. Some manual pages claimed that the error "is
returned", but RTFS and some quick testing indicate that this is wrong.

The commit message from 2003 says that:

Note our current implementation mis-matches [man pages] slightly
(error codes are stuffed into errno, where they should simply be
returned by these calls). This will be addressed shortly.

That hasn't happened in the 20 years, so we might as well make the man
pages reflect the reality.

lib: Mark up error names in man pages with .Er

s/priviledge/privilege/ in comments/log messages and fix one typo in descriptor.

s/dependend/dependent/

Sort errors.

Mention that the signal stack of the newly created lwp is reset to disabled.

branches: 1.5.14; 1.5.18;
Calling _lwp_create() with a bogus ucontext could trigger a kernel
assertion failure (and thus a crash in DIAGNOSTIC kernels). Independently
discovered by YAMAMOTO Takashi and Joel Sing.

To avoid this, introduce a cpu_mcontext_validate() function and move all
sanity checks from cpu_setmcontext() there. Also untangle the netbsd32
compat mess slightly and add a cpu_mcontext32_validate() cousin there.

Add an exhaustive atf test case, based partly on code from Joel Sing.

Should finally fix the remaining open part of PR kern/43903.

branches: 1.4.4; 1.4.8; 1.4.24;
Convert TNF licenses to new 2 clause variant

branches: 1.3.30;
Use
.In header.h
instead of
.Fd #include \*[Lt]header.h\*[Gt]
Much easier to read and write, and supported by groff for ages.
Okayed by ross.

Merge the nathanw_sa branch.

branches: 1.1.2;
file _lwp_create.2 was initially added on branch nathanw_sa.