libc: implement MIN, MAX, _setjmp, _longjmp, pselect

MIN and MAX are stinky but required by some software.

_setjmp and _longjmp are required by Lua, and come from POSIX (2017?). The difference from their _-less counterparts is that they don't save the sigmask.

select() already used ppoll(), so I renamed it to pselect(), passed the sigmask through, and made a new select() that just calls pselect().

Commit not split up because Sortix doesn't support git add -p.