Begin SystemPacketBuffer.h cleanup

[kpschoedel]

Problem

Code should use PacketBufferHandle rather than PacketBuffer *.
Some existing methods should be removed or made private so that code
does not have unnecessary access to the raw pointer.

Summary of Changes

• Consolidated PacketBuffer and PacketBufferHeader method descriptions
  into the header, and clarified which are in transition.
• Moved trivial PacketBuffer getters inline.
• Most uses of Next(), which returns a raw pointer, only checked
  for existence; added HasChainedBuffer() to replace these.
• Removed DetachTail_ForNow(), as it has no remaining callers.
• Converted TestSystemPacketBuffer.cpp to use a friend class, so that
  it can continue to use and test private methods. (Refactoring to focus
  on the PacketBufferHandle interface will follow.)

Part of issue #2707 - Figure out a way to express PacketBuffer ownership in the type system

[github-actions]

Size increase report for "nrfconnect-example-build" from 2244db6

File	Section	File	VM
chip-lighting.elf	rodata	-40	-40
chip-lighting.elf	text	-560	-560
chip-shell.elf	text	-176	-176
chip-lock.elf	rodata	-40	-40
chip-lock.elf	text	-560	-560

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

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

sections,vmsize,filesize
.debug_info,0,8429
.debug_abbrev,0,2699
.debug_line,0,480
.debug_str,0,279
.shstrtab,0,-1
.debug_aranges,0,-40
rodata,-40,-40
.symtab,0,-96
.strtab,0,-123
.debug_frame,0,-140
.debug_ranges,0,-184
.debug_loc,0,-355
text,-560,-560

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

sections,vmsize,filesize
.debug_info,0,4847
.debug_abbrev,0,1416
.debug_str,0,279
.shstrtab,0,-3
.debug_aranges,0,-40
.symtab,0,-64
.strtab,0,-77
.debug_line,0,-95
.debug_frame,0,-112
.debug_ranges,0,-128
text,-176,-176
.debug_loc,0,-439

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

sections,vmsize,filesize
.debug_info,0,8606
.debug_abbrev,0,2745
.debug_line,0,477
.debug_str,0,279
.shstrtab,0,3
.debug_aranges,0,-40
rodata,-40,-40
.symtab,0,-96
.strtab,0,-123
.debug_frame,0,-140
.debug_ranges,0,-184
.debug_loc,0,-351
text,-560,-560

[github-actions]

Size increase report for "esp32-example-build" from 2244db6

File	Section	File	VM
chip-all-clusters-app.elf	.flash.rodata	-40	-40
chip-all-clusters-app.elf	.flash.text	-432	-432

Full report output

BLOAT REPORT

Files found only in the build output:
    report.csv

Comparing ./master_artifact/chip-all-clusters-app.elf and ./pull_artifact/chip-all-clusters-app.elf:

sections,vmsize,filesize
.debug_info,0,11270
.debug_abbrev,0,1349
.debug_line,0,992
.debug_loc,0,629
.debug_str,0,342
.xt.prop._ZN4chip6System12PacketBuffer13SetDataLengthEt,0,116
.shstrtab,0,111
.debug_ranges,0,104
[Unmapped],0,40
.xt.prop._ZTVN4chip11DeviceLayer37DeviceNetworkProvisioningDelegateImplE,0,-2
.symtab,0,-16
.debug_aranges,0,-40
.flash.rodata,-40,-40
.debug_frame,0,-120
.strtab,0,-123
.flash.text,-432,-432

[bzbarsky-apple]

Suggested change

	* @note The allocation size is equal or greater than \c aAllocSize paramater to \c Create method).
	* @note The allocation size is equal to or greater than the \c aAllocSize parameter to the \c Create method).

[kpschoedel]

I'm thinking of purging this, even internal to PacketBufferHandle, because I've more than once made errors due to the asymmetry: it Frees the old mBuffer but does not AddRef the new one.

[bzbarsky-apple]

Suggested change

	* @note The allocation size is equal or to greater than \c aAllocSize parameter to the \c Create method).
	* @note The allocation size is equal to or greater than the \c aAllocSize parameter to the \c Create method).