Changes

RISC OS: update distribution ReadMe
  * Point at ROOL Universal HardDisc4
 * Shuffle origin information into 3rdParty tree
Note that the origin for ARMEABISupport is approximate: at the time of
writing, the pre-built version available there is 1.03, but we require
1.04 or later (sources for which are part of GCCSDK in the usual way).
We expect that an updated release binary will eventually be published
there, so this will do.

RISC OS: resync !Unicode resource with ROOL
This resyncs the !Unicode resource with the latest version provided by
ROOL. Additionally, we place the full copyright information into the
3rdParty tree.

RISC OS: expose !Cache licensing more obviously
Also, point at the appropriate origin for this component.

RISC OS: switch to AcornURI from ROOL
This is the version of the module provided by ROOL in their universal
HardDisc images. It is long past time to retire the cleanroom
reimplementation we used to provide (which is no longer actively
maintained).

RISC OS: update Iconv licensing information

RISC OS: s/Freenode/libera.chat/ in Help files

Update root CA bundle

RISC OS: use https in help links

RISC OS: use https in Obey files
Also remove dead link to iyonix.com: the relevant information is 
apparently now in Issue 3 of the Style Guide.

RISC OS: use https in distribution ReadMe

RISC OS: update Acorn universal boot URLs
riscos.com reorganised years ago. Follow this reorganisation
(although it may be more sensible to stop pointing at this abandonware
and suggest using the RISC OS Open universal boot, instead).

content/textplain: fix U+FFFD encoding