Rationalise and document log options somewhat.

Rationalise and document log options somewhat.

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)