flua: add freebsd.kenv, kenv(2) bindings

[bapt]

Summary

Add bindings for kenv(2) right now only get() has been created
it allows do dump into a key/value table the kernel environement if
no argument is passed, or it returns the value associated to the
provided key.

[imp]

I like this interface. Should we need to share code between userland and the loader that needs to access this interface, I think it can be trivially implemented in lua using the loader's loader.getenv for both variations. I don't think it's required that we implement that today, though.

libexec/flua/libfreebsd/kenv/freebsd.kenv.3lua

57–63

Return the
.Fa value
associated to the
.Fa key ,
if it exists, or
.Va nil
otherwise.

[bapt]

rephrase the manpage using @imp suggestion

[kevans]

libexec/flua/libfreebsd/kenv/kenv.c

50

Line after declarations for readability

87

Declaration needs to move up

91

IMO we should maintain the error interface here; that is, we return nil and a message at a minimum to be more assert-friendly

[bapt]

Address @kevans comments

[bapt]

meh

[lwhsu]

libexec/flua/libfreebsd/kenv/kenv.c

16

s/REGENTS/AUTHOR/ ?

[imp]

libexec/flua/libfreebsd/kenv/kenv.c

7

Consider dropping all this boilerplate

25

C

[bapt]

only keep SPDX headers

[bapt]

same for manpage

[kevans]

libexec/flua/libfreebsd/kenv/kenv.c

75

It just occurred to me that all of these lua_pushnil in error paths should technically be luaL_pushfail in flua/5.4. That's an alias for lua_pushnil still in 5.4, so I don't insist on fixing this now / we can clean it up later in a larger cleanup pass.