TOOLTYPE_NONNETWORK (i.e. pterm) already has "-log" (as does Unix
PuTTY), so there's no sense suppressing the synonym "-sessionlog".
Undocumented lacunae that remain:
plink accepts -sessionlog, but does nothing with it. Arguably it should.
puttytel accepts -sshlog/-sshrawlog (and happily logs e.g. Telnet
negotiation, as does PuTTY proper).
(cherry picked from commit a454399ec8d841e627d9d5e05ac977536e776754)
Conflicts:
unix/uxplink.c
windows/winplink.c
(cherry-picker's notes: the conflict was only contextual, in the Plink
help output)