Replace the EndPoint::PrepareIO() functions with stored state

[kpschoedel]

Problem

Currently, the socket-based networking implementation calls PrepareIO() on each endpoint to determine file descriptor sets for select(). This usually does redundant work (since the conditions only change on endpoint state changes) and is not usable by event-based interfaces (e.g. epoll/kqueue and libraries like libevent/libev/libuv) – see issue #5556 rework system layer event loop.

Summary of Changes

• Maintain the required flags in an endpoint member, mRequestIO, in place of recalculating them in PrepareIO(). (The points where mRequestIO is changed are the points where an event-based interface would be invoked.)
• For UDP and Raw endpoints, pass callbacks to Listen() rather than setting members separately, to avoid any possibility of races in a less synchronous environment.

Part of issue #5556 rework system layer event loop.

[github-actions]

Size increase report for "nrfconnect-example-build" from 3bb3bb9

File	Section	File	VM
chip-lock.elf	bss	0	19
chip-lock.elf	text	-16	-16
chip-lock.elf	[LOAD #3 [RW]]	0	-19
chip-lighting.elf	[LOAD #3 [RW]]	0	27
chip-lighting.elf	bss	0	5
chip-lighting.elf	text	-16	-16
chip-shell.elf	bss	0	23
chip-shell.elf	text	-16	-16
chip-shell.elf	[LOAD #3 [RW]]	0	-23

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-lock.elf and ./pull_artifact/chip-lock.elf:

sections,vmsize,filesize
.debug_loc,0,73
.debug_str,0,43
bss,19,0
.debug_abbrev,0,9
.strtab,0,9
.shstrtab,0,-1
.debug_aranges,0,-16
.debug_ranges,0,-16
text,-16,-16
[LOAD #3 [RW]],-19,0
.debug_frame,0,-44
.symtab,0,-64
.debug_line,0,-107
.debug_info,0,-1286

Comparing ./master_artifact/chip-lighting.elf and ./pull_artifact/chip-lighting.elf:

sections,vmsize,filesize
.debug_loc,0,65
.debug_str,0,43
[LOAD #3 [RW]],27,0
.debug_abbrev,0,9
.strtab,0,9
bss,5,0
.shstrtab,0,-1
.debug_aranges,0,-16
.debug_ranges,0,-16
text,-16,-16
.debug_frame,0,-44
.symtab,0,-64
.debug_line,0,-107
.debug_info,0,-1254

Comparing ./master_artifact/chip-shell.elf and ./pull_artifact/chip-shell.elf:

sections,vmsize,filesize
.debug_loc,0,65
.debug_str,0,43
bss,23,0
.debug_abbrev,0,9
.strtab,0,9
.shstrtab,0,3
.debug_aranges,0,-16
.debug_ranges,0,-16
text,-16,-16
[LOAD #3 [RW]],-23,0
.debug_frame,0,-44
.symtab,0,-64
.debug_line,0,-107
.debug_info,0,-806