Remove unused code from llexec.c and sync with main.c

[memmerto]

1) Remove unused ext_codeset variable

Specifically:

• it sets up ext_codeset from ll_langinfo()
• doesn't use it anywhere
• doesn't free it

llexec does go through the normal LL setup which configures the internal codeset (int_codeset) via init_lifelines_global() -> update_db_options() -> update_useropts(). This will set everything up for the internal / system / gedcom / report codesets, that are needed to run reports properly.

2) Add sanity checks for codeset

llexec.c does not have the sanity checks using int_codeset and transl_are_all_conversion_ok() that occur immediately after init_lifelines_postdb() in main.c, and it should.

3) Remove unused platform_init() function

This function exists but has no body and should be removed.

I'm not sure what was originally intended here, but the one thing that is missing is a call to init_arch() which is one place we do platform-ish things, so might as well add a call to that in the appropriate place.