flua: clean up lposix argument checking

The key insight here is that the luaL_check*() and luaL_opt*() functions
will happily take indexes that are larger than the stack top and print a
useful error message.

This means that there is no need to check if too few arguments have been
received prior to checking the types of individual arguments.

This patch also replaces a couple reimplementations of luaL_opt*()
functions with the luaL helpers.

References: https://www.lua.org/manual/5.4/manual.html#4.1.2
Reviewed by: emaste, kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50273