trace: fixup CTF event description at registration

trace: fixup CTF event description at registration

CTF event description is currently built by appending all fields in a
single string at trace point registration.
When dumping the metadata, this string is split again and inspected to
fixup reserved keywords and special tokens like "." or "->".

Move this fixup per field at trace point registration time so that there
is no need for inspecting / string parsing when dumping metadata.
Use dynamic allocations to remove an artificial size limit on the CTF
event description manipulations.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Sunil Kumar Kori <skori@mavell.com>